Add subobjects definition (at the data stream level) (#727)

## What does this PR do?

<!-- Mandatory
Explain here WHAT changes you made in the PR.
-->

Add support for `subobjects: false` at the data stream level. 

Here is an example:

```yaml
# From /packages/good_v3/data_stream/subobjects/manifest.yml
title: my-data-stream
type: logs
elasticsearch:
  index_template:
    mappings:
      subobjects: true
```

## Why is it important?

<!-- Mandatory
Explain here the WHY, or the rationale/motivation for the changes.
-->

Give integration developers (per data stream) access to the [subobjects](https://www.elastic.co/guide/en/elasticsearch/reference/current/subobjects.html) option in the integration's index template mappings.

Since we added the `subobjects` option in stack version 8.3, users could customize how Elasticsearch handles fields that contain dots in their names from `true` (expanded, current default) to `false` (not expanded). However, integration developers could not set this up in the integrations.

Note on per filed option: the `subobjects` option [has been available](#573) at the field level since package-spec 3.1.0. However, to make this happen at the data stream level, we needed elastic/elasticsearch#99860 to land in Elasticsearch.

## Checklist

<!-- Mandatory
Add a checklist of things that are required to be reviewed in order to have the PR approved

List here all the items you have verified BEFORE sending this PR. Please DO NOT remove any item, striking through those that do not apply. (Just in case, strikethrough uses two tildes. ~~Scratch this.~~)
-->

- [x] I have added test packages to [`test/packages`](https://github.com/elastic/package-spec/tree/main/test/packages) that prove my change is effective.
- [x] I have added an entry in [`spec/changelog.yml`](https://github.com/elastic/package-spec/blob/main/spec/changelog.yml).

## Related issues

<!-- Recommended
Link related issues below. Insert the issue link or reference after the word "Closes" if merging this should automatically close it.

- Closes #123
- Relates #123
- Requires #123
- Supersedes #123
-->
- #349
- #573
- elastic/elasticsearch#99860 (requirement)


---------

Co-authored-by: Mario Rodriguez Molins <mario.rodriguez@elastic.co>

spec/changelog.yml

@@ -2,6 +2,11 @@
 ## This file documents changes in the package specification. It is NOT a package specification file.
 ## Newer entries go at the bottom of each in-development version.
 ##
+- version: 3.2.0-next
+  changes:
+  - description: Add subobjects definition (at the data stream level)
+    type: enhancement
+    link: https://github.com/elastic/package-spec/pull/727
 - version: 3.1.3-next
   changes:
   - description: Prepare for next version

spec/integration/_dev/build/build.spec.yml

@@ -32,7 +32,7 @@ spec:
     - dependencies
 # JSON patches for newer versions should be placed on top
 versions:
-  - before: 3.1.3
+  - before: 3.2.0
     patch:
       - op: remove
         path: "/properties/dependencies/properties/ecs/properties/import_mappings/deprecated"

spec/integration/data_stream/manifest.spec.yml

@@ -279,7 +279,6 @@ spec:
                 examples:
                   - ["strict_date_optional_time","yyyy/MM/dd HH:mm:ss Z||yyyy/MM/dd Z"] # This is the default if not set.
                   - ["MM/dd/yyyy"]
-
             dynamic_templates:
               type: array
               items:
@@ -353,6 +352,64 @@ spec:
                             $ref: "./fields/fields.spec.yml#/items/properties/scaling_factor"
                           type:
                              $ref: "./fields/fields.spec.yml#/items/properties/type"
+            subobjects:
+              description: >
+                Turn the support for subobjects on or off in the mapping
+                configuration for the data stream.
+                
+                By default, it's set to `true`, enabling Elasticsearch to
+                expands fields that contain dots in their names
+                to their corresponding object structure.
+
+                Setting it to `false` restricts objects
+                from holding subobjects, enabling storing documents
+                with dot-contained field names and common prefixes.
+                
+                It is still possible to send documents that have
+                objects to a data stream that has set subobjects
+                to `false`.
+
+                Example when the subobjects mapping option is `false`:
+
+                PUT my-index-000001
+                {
+                  "mappings": {
+                    "subobjects": false 
+                  }
+                }
+
+                PUT my-index-000001/_doc/metric_1
+                {
+                  "time" : "100ms", 
+                  "time.min" : "10ms",
+                  "time.max" : "900ms"
+                }                
+
+                Before disabling subobjects, consider the following implications:
+
+                  - Nested field types cannot be used in data streams.
+                  - The subobjects mapping definition is immutable.
+                  - This setting depends on auto-flattening mappings,
+                    which comes with limitations [^1] for integration
+                    and custom mappings in data streams without
+                    subobjects.
+                  - Auto-flattening is available starting from version
+                    8.14, integrations must be on at least this
+                    version.
+
+                For comprehensive details on subobjects, refer to
+                the Elasticsearch documentation [^2].
+                
+
+                [^1]: Auto-flattening limitations are documented at:
+                https://www.elastic.co/guide/en/elasticsearch/reference/master/subobjects.html#_auto_flattening_object_mappings
+
+                [^2]: Elasticsearch documentation on subobjects:
+                https://www.elastic.co/guide/en/elasticsearch/reference/current/subobjects.html
+
+              type: boolean
+              default: true
+
         ingest_pipeline:
           description: Elasticsearch ingest pipeline settings
           type: object
@@ -515,6 +572,10 @@ spec:
   - title
 # JSON patches for newer versions should be placed on top
 versions:
+  - before: 3.2.0
+    patch:
+      - op: remove
+        path: /definitions/elasticsearch_index_template/properties/mappings/properties/subobjects
   - before: 3.0.2
     patch:
       # Required secret for variables that look like secrets.

test/packages/good_v3/data_stream/subobjects/manifest.yml

@@ -1,5 +1,9 @@
 title: subojects
 type: logs
+elasticsearch:
+  index_template:
+    mappings:
+      subobjects: true
 streams:
   - input: logfile
     vars:

test/packages/good_v3/manifest.yml

@@ -1,4 +1,4 @@
-format_version: 3.1.0
+format_version: 3.2.0
 name: good_v3
 title: Good package
 description: This package is good for format version 3