Instead of requiring that the field mapping for the timestamp field is defined
in the mapping of the composable index template for data streams, the mapping
should also be definable in the data stream definition itself, since the field
name of the data stream is already defined there.

The timestamp field mapping will be applied when a new backing index of a
data stream is being created.

When a new data stream is created and composable index templates are evaluated then
the mapping are applied in the following order:
* Optional mappings from component templates that are associated with the matching template
* Optional mappings from the matching template itself.
* Optional timestamp field mapping from data stream definition of the matching template.

If no mapping for timestamp field is defined in a component template, composable index template,
data stream definition inside a composable index template then a default mapping for the timestamp
field will be generated at data stream creation time.

Before this commit the field mapping of the timestamp field should be defined as mapping
in the composable template:

```
PUT /_index_template/logs_data_stream
{
  "index_patterns": [ "logs*" ],
  "data_stream": {
    "timestamp_field": "@timestamp"
  },
  "template": {
    "mappings": {
      "properties": {
        "@timestamp": {
          "type": "date"
        }
      }
    }
  }
}
```

With this commit, the timestamp field mapping may also be defined as part of the data stream definition:

```
PUT /_index_template/logs_data_stream
{
  "index_patterns": [ "logs*" ],
  "data_stream": {
    "timestamp_field": "@timestamp",
    "timestamp_mapping": {
      "type": "date"
    }
  }
}
```

The field mapping is now also optional:

```
PUT /_index_template/logs_data_stream
{
  "index_patterns": [ "logs*" ],
  "data_stream": {
    "timestamp_field": "@timestamp"
  }
}
```

In the latter case the following mapping snippet is used as default:
{"type": "date"}

Closes to elastic#58583

…as specified

…_template

…_template