refactor: avoid use of async_trait for RoomIdentityProvider

[bnjbvr]

This is an 8 seconds (out of 22) decrease of the matrix-sdk compile times. BoxFuture ftw.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.41%. Comparing base (bc8c4f5) to head (872b677).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/room/mod.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4442      +/-   ##
==========================================
- Coverage   85.42%   85.41%   -0.02%     
==========================================
  Files         283      283              
  Lines       31556    31559       +3     
==========================================
- Hits        26958    26955       -3     
- Misses       4598     4604       +6     

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[andybalaam]

Wow, thank you!