feat(event cache): don't strip a bundled thread relation when storing it in the event cache

[bnjbvr]

We may be reusing the bundled thread info as a location where to save the thread summary in a near future, so it's useful to keep it from sync, for the sake of our experiments.

Part of #4869.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.83%. Comparing base (a27d6e2) to head (783ba1d).
Report is 82 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/room/mod.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4941      +/-   ##
==========================================
+ Coverage   85.82%   85.83%   +0.01%     
==========================================
  Files         323      323              
  Lines       35512    35514       +2     
==========================================
+ Hits        30478    30485       +7     
+ Misses       5034     5029       -5     

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[bnjbvr]

Drafting; need to think about it a bit more.

[bnjbvr]

Not the right direction: we'll use a separate ThreadsSummary for more flexibility.