MSC3967: Do not require UIA when first uploading cross signing keys

[S7evinK]

Playing around with Copilot, tests are generated.

Requires matrix-org/gomatrixserverlib#444

[S7evinK]

The tests are separated for readability, as I think table tests would make them harder to read.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.31034% with 12 lines in your changes missing coverage. Please review.

Project coverage is 49.38%. Comparing base (add73ec) to head (e05504c).

Files with missing lines	Patch %	Lines
clientapi/routing/key_crosssigning.go	78.94%	11 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3471      +/-   ##
==========================================
- Coverage   49.39%   49.38%   -0.01%     
==========================================
  Files         524      524              
  Lines       59737    59771      +34     
==========================================
+ Hits        29507    29520      +13     
- Misses      26748    26766      +18     
- Partials     3482     3485       +3     

Flag	Coverage Δ
unittests	49.38% <79.31%> (-0.01%)	⬇️

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