Refactor Janitor service GC actor

[evanxg852000]

Refactor the Garbage collection actor in Janitor service to directly work on dangling splits.

• We want to have a new method on the Metastore that return the list of garbage collectable splits in all registered indexes.
• We also want the list_splits method of the metastore to have a filter on node_id as we have some common splits operations (clean local split store) done at node level.

Related: https://github.com/quickwit-oss/quickwit/issues/1915