Make callback interactions with ClientSessionDelegate async

[zzorba]

See issue: #4516

The client we are using the matrix-rust-sdk (react-native) does not offer synchronous access to the system keychain. In order to make this easier, this change adjusts ClientSessionDelegate to be a CallbackInterface with async methods for storing and retrieving. Retrieving is the only on that strictly speaking needs to be async, but it felt like symmetry was warranted here in case the save operation were to fail at the system level.

Public API changes documented in changelogs (optional)
Signed-off-by: Daniel Salinas

[zzorba]

I do not understand the compilation error, it seems to only be flagged in tests, and I can't figure out how to run the failing step locally, making debugging very challenging.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.40%. Comparing base (2bd8c56) to head (b11f0b7).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/oidc/cross_process.rs	33.33%	2 Missing ⚠️
crates/matrix-sdk/src/oidc/mod.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4549   +/-   ##
=======================================
  Coverage   85.40%   85.40%           
=======================================
  Files         285      285           
  Lines       32213    32216    +3     
=======================================
+ Hits        27511    27514    +3     
  Misses       4702     4702           

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