feat(send queue): automatically subscribe when replying in a thread

[bnjbvr]

This adds a manual thread subscription when a user is replying to a thread, via the send queue, according to the semantics of MSC4306.

Probably needs some kind of centralized mechanism to automatically subscribe when sending a message in the send queue, for both the send queue and the event cache.

Part of #4869.

[codecov]

Codecov Report

❌ Patch coverage is 84.86842% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.56%. Comparing base (e2f0b4f) to head (ed250ba).
⚠️ Report is 34 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/room/threads.rs	80.70%	7 Missing and 4 partials ⚠️
crates/matrix-sdk/src/event_cache/room/mod.rs	84.21%	2 Missing and 4 partials ⚠️
crates/matrix-sdk/src/send_queue/mod.rs	75.00%	3 Missing and 1 partial ⚠️
crates/matrix-sdk-common/src/serde_helpers.rs	85.71%	0 Missing and 1 partial ⚠️
crates/matrix-sdk/src/room/mod.rs	95.65%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5520      +/-   ##
==========================================
- Coverage   88.56%   88.56%   -0.01%     
==========================================
  Files         340      340              
  Lines       93659    93784     +125     
  Branches    93659    93784     +125     
==========================================
+ Hits        82951    83061     +110     
- Misses       6574     6583       +9     
- Partials     4134     4140       +6     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5520 will not alter performance

_{Comparing bnjbvr/auto-sub-send (ed250ba) with main (e2f0b4f)}

Summary

✅ 31 untouched benchmarks

[bnjbvr]

Fold in the other PR