Threads: List of threads of the room

This will act act as an SDK side meta issue for all the tasks relating to **properly** rendering a room's list of threads. [meta](https://github.com/element-hq/element-meta/issues/2954)

Previous work in this regard includes:
* matrix-sdk crate level methods for interacting with the backend and the [Thread Lists API](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3856-threads-list-api.md) in https://github.com/matrix-org/matrix-rust-sdk/pull/4973
* exposing said methods through FFI in https://github.com/matrix-org/matrix-rust-sdk/pull/5953

Unfortunately #5953 only theoretically exposed types as `uniffi::Object`s do not automatically publish fields over FFI, [so this change](https://github.com/matrix-org/matrix-rust-sdk/pull/6278/changes/9175220c60b933a4a355bb44ec7ad9c2b02e8079) was required.

More so, exposing direct sync timeline events (`TimelineEvent`) isn't particularly useful for final clients which generally interact with UI crate level types and aggregations. #6278 came on top to convert sync types to UI types similar to the implementation found in `LatestEvents`.

While these are all welcomed improvements **we still need the following**:

- [x] Expose the latest in-thread event and its sender profile on the ThreadListItem
- [x] Expose the number of replies in the thread
- [x] Expose unread/mention/notification states for each thread
- [ ] Aggregate unreads and expos whether there are **any** unread threads [meta](https://github.com/element-hq/element-meta/issues/2952)
- [x] Put the thread list behind a reactive interface within a new service, let's call it the `ThreadListService` (UI crate level)
- [x] Have the `ThreadListService` hold state and handle pagination
- [ ] Hook up the service to Room and EventCache updates so thread list items and the list itself automatically update
- [ ] First hit the EventCache before hitting the backend when loading thread lists

At the end of all of this we will have an API can easily power the following UI: