feat(ui): introduce a new ThreadListService

[stefanceriu]

ThreadListService is the FFI-facing wrapper around [matrix_sdk_ui::timeline::thread_list_service::ThreadListService].
It maintains an observable list of [ThreadListItem]s and exposes a pagination state publisher, making it straightforward to build reactive UIs on top of the thread list.

This PR also removes the existing Room:load_thread_list method in favor of this new service.

Part of #6290

[codecov]

Codecov Report

❌ Patch coverage is 89.55823% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.88%. Comparing base (4ad046d) to head (0f2007b).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../matrix-sdk-ui/src/timeline/thread_list_service.rs	90.28%	23 Missing and 1 partial ⚠️
crates/matrix-sdk-ui/src/timeline/traits.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6311      +/-   ##
==========================================
+ Coverage   89.81%   89.88%   +0.06%     
==========================================
  Files         376      376              
  Lines      102461   102646     +185     
  Branches   102461   102646     +185     
==========================================
+ Hits        92026    92261     +235     
+ Misses       6855     6809      -46     
+ Partials     3580     3576       -4     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing stefan/threadListService (0f2007b) with main (daa1120)}