Add new DLM Frozen Tier Transition execution plugin and service

[lukewhiting]

This PR adds a new X-Pack plugin for the DLM frozen tier and creates an execution framework for that transition.

Essentially there's a scheduler thread that checks for new indices with the "To transition" metadata maker every X minutes.

If it finds any, it checks if that index is already being transitioned and that there's available threads to execute the transition. If there is, it starts the transition on a thread in it's managed, fixed size thread pool. If not, the index is skipped if executing or all remaining indices are skipped if the thread pool is at capacity.

All this is gated behind a feature flag.

Also depends on #144511 to make Datas Streams plugin extensible / module exposed.

Fixes elastic/elasticsearch-team#2433