feat(crypto): align NUT-20/29 signatures with spec change

[a1denvalu3]

Aligns NUT-20/29 batch minting signatures to the spec changes in cashubtc/nuts pull request 375, while retaining fallback legacy signature verification for older wallets.

[a1denvalu3]

This aligns nutshell with the NUT-20/29 spec changes defined in: cashubtc/nuts#375

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.19%. Comparing base (71df49b) to head (86564db).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/core/nuts/nut20.py	82.14%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1054      +/-   ##
==========================================
+ Coverage   75.15%   75.19%   +0.03%     
==========================================
  Files         111      111              
  Lines       12372    12395      +23     
==========================================
+ Hits         9298     9320      +22     
- Misses       3074     3075       +1     

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