LNbits compatibility: use bip32 from embit if bip32 lib is not available

[callebtc]

For LNbits. This will not be merged into the nutshell main project.

• uses embit if bip32 is not available
• uses multiple db transactions during mint migration because lnbits db locks otherwise

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 67.86%. Comparing base (e040479) to head (a3c4399).
Report is 1 commits behind head on main.

Files	Patch %	Lines
cashu/core/crypto/keys.py	53.84%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
- Coverage   67.89%   67.86%   -0.04%     
==========================================
  Files          76       76              
  Lines        6815     6827      +12     
==========================================
+ Hits         4627     4633       +6     
- Misses       2188     2194       +6     

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