[Fleet] added _meta field has_experimental_data_stream_indexing_features

[juliaElastic]

Summary

Closes #150917

Added has_experimental_data_stream_indexing_features meta field to indicate that component/index template has these setting turned on.

The component template has this field set when synthetic_source or the doc_value_only settings are enabled.
The index template has the field set when tsdb setting is enabled.

Open question:

• Alternatively we could move the meta field to only be in the component template, also for tsdb. I'm not sure which would be more intuitive (tsdb only makes changes to the index template, the other settings in component template).
  @kpollich WDYT?

To test:

• enable experimentalDataStreamSettings feature flag
• add a System integration policy and enable one of the experimental features on one data stream e.g. Doc only value
• Check the corresponding component template in Stack Management that is has the new _meta field.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Alternatively we could move the meta field to only be in the component template, also for tsdb. I'm not sure which would be more intuitive (tsdb only makes changes to the index template, the other settings in component template).
@kpollich WDYT?

I think it makes the most sense to add the _meta property to whichever template directly includes the experimental functionality. So for TSDB, including the _meta property in the index template makes sense. For the others, including the _meta property in the component templates makes sense. So this implementation LGTM.

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: a41fa95

Failed CI Steps

• FTR Configs #49

Test Failures

• [job] [logs] FTR Configs #49 / Lists plugin Lists API "after all" hook for "should return a 400 if endpoint exception entry has disallowed field"
• [job] [logs] FTR Configs #49 / Lists plugin Lists API "before all" hook for "should return a 400 if an endpoint exception item with a list-based entry is provided"

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @juliaElastic