refactor(timeline): Make RoomDataProvider provide Decryptor to simplify redecryption

[andybalaam]

Part of the work on #5474

This should help us (in future changes) to call into the decryptor thread from code that adds events to the timeline.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.57%. Comparing base (a9ce1c6) to head (2d02de1).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5517   +/-   ##
=======================================
  Coverage   88.57%   88.57%           
=======================================
  Files         340      340           
  Lines       93694    93665   -29     
  Branches    93694    93665   -29     
=======================================
- Hits        82989    82964   -25     
+ Misses       6573     6571    -2     
+ Partials     4132     4130    -2     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5517 will not alter performance

_{Comparing andybalaam/hold-decryptor-in-timelinecontroller (2d02de1) with main (a9ce1c6)}

Summary

✅ 31 untouched benchmarks

[bnjbvr]

LGTM, thanks!

This is just a refactor, with no changes in behavior, at this point, right?

Can you tweak the PR name so that it also follows the commit message convention, please?

[andybalaam]

This is just a refactor, with no changes in behavior, at this point, right?

Yes

Can you tweak the PR name so that it also follows the commit message convention, please?

Done, thanks.