Relates to #132818

Indexing features like synthetic _source and time_series have common incompatibilities with many non-metrics datastreams. This can cause confusion for users when they try to opt-in to these experimental features.

Since these UI toggles are intended as a largely internal, experimental option for integration maintainers to test and try out these index settings, we should gate the UI options behind a feature flag.

Implementation

• xpack.fleet.enableExperimental: ['experimentalDataStreamSettings'] flag is supported in kibana.yml
• When the above flag is true, the existing "Experimental Indexing Features" section in the policy editor is displayed
• When false, the section is not displayed
• The experimental settings field in the create/edit package policy API can still be supported even if the flag is false - to allow for overrides in extreme cases