Move SendJoin logic to GMSL

[devonh]

Moves the core matrix logic for handling the send_join endpoint over to gmsl.

[codecov]

Codecov Report

Patch coverage: 0.83% and project coverage change: -0.10 ⚠️

Comparison is base (027a9b8) 66.25% compared to head (efa4db3) 66.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3084      +/-   ##
==========================================
- Coverage   66.25%   66.15%   -0.10%     
==========================================
  Files         497      497              
  Lines       53793    53724      -69     
==========================================
- Hits        35640    35542      -98     
- Misses      14573    14608      +35     
+ Partials     3580     3574       -6     

Flag	Coverage Δ
unittests	49.57% <0.83%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
federationapi/routing/join.go	27.79% <0.00%> (-19.02%)	⬇️
federationapi/routing/routing.go	72.60% <0.00%> (-2.91%)	⬇️
roomserver/api/api.go	100.00% <ø> (ø)
roomserver/internal/query/query.go	59.44% <100.00%> (-0.42%)	⬇️

... and 16 files with indirect coverage changes

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