Sliding Sync: Add cache to `get_tags_for_room(...)` (#17730)

Add cache to `get_tags_for_room(...)`

This helps Sliding Sync because `get_tags_for_room(...)` is going to be
used in https://github.com/element-hq/synapse/pull/17695

Essentially, we're just trying to match `get_account_data_for_room(...)`
which already has a tree cache.