feat(pinned events): persist the pinned events timeline

[bnjbvr]

This brings some nice benefits:

• reloading the previous list of pinned events from storage, which gives us…
• …offline access for pinned events
• better support for automatic redecryption of pinned events

Part of #5954.

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing bnjbvr/pinned-events-linked-chunk (7f1f465) with main (52014dc)}

[codecov]

Codecov Report

❌ Patch coverage is 78.58770% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.78%. Comparing base (70dd020) to head (7f1f465).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...s/matrix-sdk/src/event_cache/room/pinned_events.rs	74.93%	64 Missing and 26 partials ⚠️
crates/matrix-sdk/src/event_cache/room/mod.rs	89.65%	0 Missing and 3 partials ⚠️
...rates/matrix-sdk-ui/src/timeline/controller/mod.rs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6085      +/-   ##
==========================================
- Coverage   89.84%   89.78%   -0.07%     
==========================================
  Files         363      363              
  Lines      100656   100973     +317     
  Branches   100656   100973     +317     
==========================================
+ Hits        90439    90655     +216     
- Misses       6691     6762      +71     
- Partials     3526     3556      +30     

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