refactor(room): move PinnedEventsRoom::load_event_with_relations to the Room object

[bnjbvr]

The method is kept on the pinned loader trait at the moment, because it's too inconvenient to remove it quite yet. This will happen in a subsequent PR.

Part of #5954. Prerequisite for #6085.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.83%. Comparing base (40c6c33) to head (93a0ebc).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/room/mod.rs	86.95%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6108      +/-   ##
==========================================
- Coverage   89.84%   89.83%   -0.01%     
==========================================
  Files         363      363              
  Lines      100638   100656      +18     
  Branches   100638   100656      +18     
==========================================
+ Hits        90420    90429       +9     
- Misses       6690     6702      +12     
+ Partials     3528     3525       -3     

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing bnjbvr/load-event-with-relations (93a0ebc) with main (40c6c33)}

Summary

✅ 50 untouched benchmarks