Add Cashu wallet backend

[callebtc]

Adds Cashu wallet backend through the REST API.

Depends on https://github.com/cashubtc/nutshell/pull/318

[codecov[bot]]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (8eabf53) 59.29% compared to head (bea9e43) 59.16%.

Files	Patch %	Lines
lnbits/wallets/cashu.py	33.33%	28 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2063      +/-   ##
==========================================
- Coverage   59.29%   59.16%   -0.13%     
==========================================
  Files          60       61       +1     
  Lines        8942     8987      +45     
==========================================
+ Hits         5302     5317      +15     
- Misses       3640     3670      +30     

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[dni]

@callebtc the required PR seems to be merged, can you take a look again?

[motorina0]

@callebtc

• is this PR completed
• is there a cashu docker image that we can use in our regtest