crypto: serialize keys with alg-prefixed hex strings

[divarvel]

add from_bytes_string() and to_bytes_string() on PublicKey and PrivateKey, which look like the datalog syntax (ed25519/<hex bytes>), so that the key algorithm is explicit.

This will avoid confusions at program boundaries, when the keys must be read from strings (eg in biscuit-cli or when reading keys from environment variables).

I chose to implement it only on the top level structures (ie not in ed25519 and p256 modules), and in new methods to avoid breaking existing code.

[codspeed-hq]

CodSpeed Performance Report

Merging #261 will not alter performance

_{Comparing explicit-key-prefix (e6e2484) with main (f4473ee)}

Summary

✅ 12 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.70%. Comparing base (f4473ee) to head (e6e2484).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
+ Coverage   67.39%   67.70%   +0.31%     
==========================================
  Files          39       39              
  Lines        7235     7246      +11     
==========================================
+ Hits         4876     4906      +30     
+ Misses       2359     2340      -19     

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