Ability to run `_forcemerge?only_expunge_deletes=true` via schedule in ILM

With https://github.com/elastic/elasticsearch/pull/77478 being introduced, it might be useful if ILM had the ability to call `_forcemerge?only_expunge_deletes=true` periodically. This would allow users to avoid custom tooling if they have a use case where `_forcemerge?only_expunge_deletes=true` is performed regularly to free up disk space.