refactor: Move FederationRequest/InviteV2 to fclient

[kegsay]

Also delete dead code.

[codecov]

Codecov Report

Patch coverage: 55.96% and project coverage change: +0.12 🎉

Comparison is base (3cf4cd9) 52.62% compared to head (2113ec2) 52.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #364      +/-   ##
==========================================
+ Coverage   52.62%   52.74%   +0.12%     
==========================================
  Files          37       37              
  Lines        5893     5879      -14     
==========================================
  Hits         3101     3101              
+ Misses       2495     2481      -14     
  Partials      297      297              

Impacted Files	Coverage Δ
authstate.go	25.75% <0.00%> (ø)
backfill.go	62.50% <ø> (ø)
edu.go	0.00% <ø> (ø)
eventversion.go	53.48% <0.00%> (ø)
fclient/client.go	24.85% <0.00%> (ø)
fclient/crosssigning.go	0.00% <ø> (ø)
fclient/federationtypes.go	24.62% <ø> (ø)
json.go	82.04% <0.00%> (ø)
keys.go	0.00% <0.00%> (ø)
spec/base64.go	83.33% <ø> (ø)
... and 19 more

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