New server registration validation flow

[Firehed]

This replaces $server->setRegisterRequest($request)->register($response) with $server->validateRegistration($request, $response)

[codecov]

Codecov Report

Merging #30 (a564f58) into master (d08126c) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #30   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       134       135    +1     
===========================================
  Files             21        21           
  Lines            545       547    +2     
===========================================
+ Hits             545       547    +2     

Impacted Files	Coverage Δ
src/Server.php	100.00% <100.00%> (ø)

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