crypto: new VerificationLevel::MismatchedSender

[richvdh]

If we believe that the sender of a room event has been spoofed, use a dedicated VerificationLevel, possibly allowing us to treat such events more carefully.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.15%. Comparing base (404a982) to head (0aece69).
Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5219      +/-   ##
==========================================
- Coverage   85.16%   85.15%   -0.02%     
==========================================
  Files         329      329              
  Lines       36923    36924       +1     
==========================================
- Hits        31446    31441       -5     
- Misses       5477     5483       +6     

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