[meta] Latest Event API, for Threads

Theseus and Ariadne (Angelika Koffmann)

Extracted from https://github.com/matrix-org/matrix-rust-sdk/issues/4112 to reduce its scope.

The goal is to add Thread support in the LatestEvents API.

Plan

Note: Thread's events aren't persisted in the Event Cache yet. Until it's done, Latest Events for Threads are likely to be LatestEventValue::None, but we are still implementing that, so it's thread-aware™️ thread-ready™️.

• From the Event Cache:
  • Add RoomEventCacheGenericUpdate support for threads + thread pagination
  • Add ThreadEventCache::rfind_map_event_in_memory_by
  • Add RoomEventCache::thread to get a ThreadEventCache
  • Update LatestEventValueBuilder::new_remote_with_power_levels to take into account Self::thread_id.
• From the Send Queue:
  • Nothing in particular, just extract the thread root from Thread::event_id to see if it matches the targeted thread.
• See if it closes https://github.com/matrix-org/matrix-rust-sdk/issues/5702.
• See where to store the LatestEventValue, probably in ThreadInfo or ThreadSummary:
  • https://github.com/matrix-org/matrix-rust-sdk/issues/6013
  • https://github.com/matrix-org/matrix-rust-sdk/pull/6893

Start with the Event Cache items in the plan, especially RoomEventCacheGenericUpdate, ThreadEventCache::rfind_map_event_in_memory_by, and LatestEventValueBuilder::new_remote_with_power_levels, then review the Send Queue note about Thread::event_id. Done means LatestEvents is thread-aware, the listed cache and pagination work is covered, and the LatestEventValue storage location is settled in ThreadInfo or ThreadSummary.