Have the library handle challenge management

[Firehed]

The examples so far have all used sessions to manage the active challenges, but not all applications are stateful in this way - namely, most APIs will not be session-based.

Instead, this creates a new ChallengeManagerInterface that handles this for applications. For now there's a single implementation that's still session-based, though (via #30 which I'm reworking) other implementations will be provided (e.g. a cache pool).

The majority of the change here is updating examples and adding tests. Note that this would be a BC break but since the library is still pre-1.0 it's not a concern for practical purposes.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ae5e600) 77.30% compared to head (75a5986) 78.16%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #35      +/-   ##
============================================
+ Coverage     77.30%   78.16%   +0.86%     
- Complexity      140      147       +7     
============================================
  Files            21       22       +1     
  Lines           467      490      +23     
============================================
+ Hits            361      383      +22     
- Misses          106      107       +1     

Files	Coverage Δ
src/CreateResponse.php	91.89% <100.00%> (+0.98%)	⬆️
src/GetResponse.php	94.73% <100.00%> (+0.50%)	⬆️
src/SessionChallengeManager.php	92.85% <92.85%> (ø)

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