feat(threads): implement msc4308 companion endpoint

[bnjbvr]

This bumps Ruma (to an unstable branch, because we need ruma/ruma#2200 for tests, duh), which brings us a few documented test fixes.

In addition to this, this implements the support for the companion endpoint of MSC4308, which allows to back-paginate thread subscriptions, if the sync response contained only a small subset of the entire change set.

Part of #5038.

[codecov]

Codecov Report

❌ Patch coverage is 98.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.62%. Comparing base (99af951) to head (0e74ffd).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/client/mod.rs	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5590   +/-   ##
=======================================
  Coverage   88.61%   88.62%           
=======================================
  Files         341      341           
  Lines       95413    95466   +53     
  Branches    95413    95466   +53     
=======================================
+ Hits        84550    84602   +52     
  Misses       6646     6646           
- Partials     4217     4218    +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5590 will not alter performance

_{Comparing bnjbvr/msc4308-companion-endpoint (0e74ffd) with main (99af951)}

Summary

✅ 49 untouched benchmarks

[Hywan]

It looks good, thanks for the small patches!