Session verification request support

[stefanceriu]

This PR introduced FFI level API support for interacting with incoming session verification requests. Best reviewed commit by commit

• it exposes a new did_receive_verification_request SessionVerificationController delegate method that includes information about the incoming request/device/sender
• the client can acknowledge_verification_request to get informed about it getting accepted somewhere else or cancelled
• the client can also accept_verification_request to start interacting with it

This PR also internally switches to the VerificationRequest::changes publisher, fixes a couple of invalid VerificationMachine tests and starts storing requesting DeviceData directly in VerificationRequestState. Requested

Relates to element-hq/element-meta/issues/2464, fixes #3595.

[stefanceriu]

Please double check the commit messages, I tried making them fit the new contributing guidelines but not sure I nailed it.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 20 lines in your changes missing coverage. Please review.

Project coverage is 84.89%. Comparing base (ca1d829) to head (61b6518). Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
...tes/matrix-sdk-crypto/src/verification/requests.rs	64.86%	13 Missing :warning:
...ates/matrix-sdk-crypto/src/verification/machine.rs	36.36%	7 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4153      +/-   ##
==========================================
+ Coverage   84.85%   84.89%   +0.03%     
==========================================
  Files         269      269              
  Lines       28916    28902      -14     
==========================================
- Hits        24537    24536       -1     
+ Misses       4379     4366      -13     

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[richvdh]

Taking myself off the reviewer list here because it looks like @poljar already has context on it and will do a better job than me anyway

[poljar]

Oh about the commits, some of them you got right some of them not.

fix(crypto): ... is correct, crypto(verification) is not. In the second case you should have used fix(verification) or something along those lines.

The format is documented here: https://www.conventionalcommits.org/en/v1.0.0/.