Add custom metadata support to data steams.

[martijnvg]

Composable index template may hold custom metadata. This change adds behaviour that
when a data stream gets created the custom metadata of the matching composable index
template is copied to new data stream. The get data stream api can then be used to
view the custom metadata.

Example:

PUT /_index_template/my-logs-template
{
  "index_patterns": [ "logs-*" ],
  "data_stream": { },
  "template": {
      "settings": {
          "index.number_of_replicas": 0
      }
  },
  "_meta": {
      "managed": true
  }
}

PUT /_data_stream/logs-myapp

GET /_data_stream

The get data stream api then yields the following response:

{
    "data_streams": [
        {
            "name": "logs-myapp",
            "timestamp_field": {
                "name": "@timestamp"
            },
            "indices": [
                {
                    "index_name": ".ds-logs-myapp-000001",
                    "index_uuid": "3UaBxM3mQXuHR6qx0IDVCw"
                }
            ],
            "generation": 1,
            "_meta": {
                "managed": true
            },
            "status": "GREEN",
            "template": "my-logs-template"
        }
    ]
}

Closes #59195

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Data streams)

[danhermann]

LGTM

[yuliacech]

Hi @martijnvg , thanks a lot for adding this! Tested locally and got the data stream's meta data in response.