feat(threads): add methods to list a room's threads + list all related events

[bnjbvr]

The tests show how to use them for listing threads (Room::list_threads()), and the events within a thread (Room::relations()).

This should be sufficient to implement an in-memory (viz. not persisted on disk) RoomThreadEventCache 👀 according to the plan described in #4869.

Part of #4869.

[codecov]

Codecov Report

Attention: Patch coverage is 68.22430% with 34 lines in your changes missing coverage. Please review.

Project coverage is 85.83%. Comparing base (c3ed8b9) to head (12c280b).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/test_utils/mocks/mod.rs	55.17%	26 Missing ⚠️
crates/matrix-sdk/src/room/messages.rs	80.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4973      +/-   ##
==========================================
- Coverage   85.89%   85.83%   -0.07%     
==========================================
  Files         325      325              
  Lines       35731    35838     +107     
==========================================
+ Hits        30692    30761      +69     
- Misses       5039     5077      +38     

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[poljar]

It's a bit strange that the last commit introduces test helpers that were used in the previous commit, there's also a typo in the last commit message.

Overall looks good, I would liked some examples for some of those new methods but alas.

I'm going to fix the typo and commit ordering myself.