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 elastic#59195