Make Challenge class

[Firehed]

This is groundwork for adjustments to the authentication flow

[codecov]

Codecov Report

Merging #31 (9978525) into master (4b7c8c6) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #31   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       135       138    +3     
===========================================
  Files             21        22    +1     
  Lines            547       554    +7     
===========================================
+ Hits             547       554    +7     

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

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