Add force-merge step to searchable snapshots action

[DaveCTurner]

It's preferable when searching a snapshot that the snapshotted index was force-merged to a single segment first. ILM lets you configure a suitable force-merge action in an earlier phase but does not actively encourage its use, and we also see a need to support occasional writes in the warm phase so that the index may no longer be force-merged by the time it is moved to the cold phase.

We discussed this today and decided it would be good to add a force-merge step to the searchable snapshots action. This step would be enabled by default but we'd include an option to disable it. Force-merging an already-force-merged index is a no-op so it's ok always to run this step.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/ILM+SLM)