Add false_allow_templates as a dynamic mapping option

[bruce-hong-glean]

Description

This PR adds the dynamic: false_allow_templates option to the schema. This creates mappings for any field that matches properties or dynamic_templates, and ignores any fields that do not (as opposed to rejecting altogether).

Example from linked issue, the usage is:

1. Create the index:

{
    "properties": {
        "url": { "type": "keyword" },
    },
    "dynamic": "false_allow_templates",
    "dynamic_templates": [
        {
            "dates": {
                "match": "date_*",
                "mapping": {
                    "type": "date",
                },
            },
        }
    ]
}

2. url matches an explicit property and the fields date_timestamp and date_timezone math the dynamic template:

POST index1/_doc/1
{
  "url": "www.example.com",
  "date_timestamp": "...",
  "date_timezone": "...",
  "misc_field": "..."
}

3. Produces the mapping:

{
    "properties": {
        "url": { "type": "keyword" },
        "date_timestamp": { "type": "date" },
        "date_timezone": { "type": "date" }
    }
}

Related Issues

Resolves #18617

Check List

• Functionality includes testing.
• API changes companion pull request created
• Public documentation issue/PR created

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