This commit adds a new querystring parameter on the following APIs:
- Index
- Update
- Bulk
- Create Index
- Rollover
These APIs now support a `?prefer_v2_templates=true|false` flag. This flag changes the preference
creation to use either V2 index templates or V1 templates. This flag defaults to `false` and will be
changed to `true` for 8.0+ in subsequent work.
Additionally, setting this flag internally sets the `index.prefer_v2_templates` index-level setting.
This setting is used so that actions that automatically create a new index (things like rollover
initiated by ILM) will inherit the preference from the original index. This setting is dynamic so
that a transition from v1 to v2 templates can occur for long-running indices grouped by an alias
performing periodic rollover.
This also adds support for sending this parameter to the High Level Rest Client.
Relates to #53101
9eddd2bcc9