feat(sdk): Introduce the new LatestEvents API

[Hywan]

This patch is the first part of the new LatestEvents API. It contains
the “framework”, i.e. the structure, tasks, logic to make it work, but
no latest events are computed for the moment.

The Latest Events API provides a lazy, reactive and efficient way to
compute the latest event for a room or a thread.

The latest event represents the last displayable and relevant event
a room or a thread has been received. It is usually displayed in a
summary, e.g. below the room title in a room list.

The entry point is LatestEvents. It is preferable to get a reference
to it from Client::latest_events, which already plugs everything to
build it. LatestEvents is using the EventCache and the SendQueue
to respectively get known remote events (i.e. synced from the server),
or local events (i.e. ones being sent).

Laziness

LatestEvents is lazy, it means that, despites LatestEvents
is listening to all EventCache or SendQueue updates, it will
only do something if one is expected to get the latest event for a
particular room or a particular thread. Concretely, it means that
until LatestEvents::for_room is called for a particular room, no
latest event will ever be computed for that room (and similarly with
LatestEvents::for_thread).

If one is no longer interested to get the latest event for a
particular room or thread, the LatestEvents::forget_room and
LatestEvents::forget_thread methods must be used.

Reactive

LatestEvents is designed to be reactive. Once one has fetched a
LatestEvent, it is recommended to immediately subscribe to its updates
with LatestEvent::subscribe.

---

• Address [meta] Latest Event API, the missing one #4112

[codecov]

Codecov Report

Attention: Patch coverage is 90.92784% with 44 lines in your changes missing coverage. Please review.

Project coverage is 88.74%. Comparing base (e0ab16f) to head (4d8c194).
Report is 47 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/latest_events/mod.rs	91.46%	31 Missing and 7 partials ⚠️
...rates/matrix-sdk/src/latest_events/latest_event.rs	53.84%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5298      +/-   ##
==========================================
- Coverage   90.20%   88.74%   -1.47%     
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  Files         334      331       -3     
  Lines      105120    89144   -15976     
  Branches   105120    89144   -15976     
==========================================
- Hits        94827    79107   -15720     
+ Misses       6233     6226       -7     
+ Partials     4060     3811     -249     

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