Support base64url access for challenges; adjust legacy credential format

[Firehed]

This reduces the need for RP servers to use internal methods by directly exposing the challenge as base64url, which is used by the (limited support) native formats as noted in #48 (a tool to produce the entire format will come as well!).

Additionally, this matches up the storage id formats for both credential formats, which as-is would not function as expected (new authorizations would return a v2, and subsequently could fail to find an existing matching v1 format depending on the lookup approach)

Warning

BC BREAK: this changes the storage identifier for the older credential format. This was noted in tests as a pre-1.0 possibility.

Fixes #59

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8922c02) 85.23% compared to head (fb84f46) 85.27%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #61      +/-   ##
============================================
+ Coverage     85.23%   85.27%   +0.03%     
- Complexity      253      256       +3     
============================================
  Files            27       27              
  Lines           813      815       +2     
============================================
+ Hits            693      695       +2     
  Misses          120      120              

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