feat(crypto): align NUT-20/29 signatures with spec and add test vectors

[a1denvalu3]

This PR aligns NUT-20 and NUT-29 signature validation and generation with the latest spec amendments, and implements/verifies the corresponding test vectors in test_crypto.py.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.27%. Comparing base (8d23ea9) to head (4ab4593).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/core/nuts/nut20.py	83.33%	5 Missing ⚠️
cashu/mint/ledger.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1057   +/-   ##
=======================================
  Coverage   75.27%   75.27%           
=======================================
  Files         111      111           
  Lines       12431    12458   +27     
=======================================
+ Hits         9357     9378   +21     
- Misses       3074     3080    +6     

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