Add pem/der loaders to PublicKey

[Geal]

rebase of #212 over main

PEM key loading is only supported for ed25519 right now (so the relevant methods on the common KeyPair etc types are removed). Do we add it for p256 curves?

[codspeed-hq]

CodSpeed Performance Report

Merging #265 will not alter performance

_{Comparing geal/pem-fixes (0fc8a57) with main (8dabc87)}

Summary

✅ 12 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 96.72131% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.61%. Comparing base (8dabc87) to head (0fc8a57).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
biscuit-auth/src/crypto/mod.rs	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
+ Coverage   67.23%   68.61%   +1.38%     
==========================================
  Files          39       39              
  Lines        7407     7424      +17     
==========================================
+ Hits         4980     5094     +114     
+ Misses       2427     2330      -97     

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[divarvel]

PEM key loading is only supported for ed25519 right now (so the relevant methods on the common KeyPair etc types are removed). Do we add it for p256 curves?

yes, that would be great to have PEM support for both algorithms and for private and public keys