[Wallet] Define responses for API

[sihamon]

Define responses of wallet API using pydantic's BaseModel and fastapi's response_model.

[codecov]

Codecov Report

Patch coverage: 89.53% and project coverage change: -0.24 ⚠️

Comparison is base (f0735cf) 55.38% compared to head (49a455e) 55.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
- Coverage   55.38%   55.15%   -0.24%     
==========================================
  Files          41       42       +1     
  Lines        3286     3385      +99     
==========================================
+ Hits         1820     1867      +47     
- Misses       1466     1518      +52     

Impacted Files	Coverage Δ
cashu/wallet/api/router.py	69.85% <75.67%> (-0.33%)	⬇️
cashu/wallet/api/responses.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[callebtc]

Excellent this should make it a lot easier to follow along and generate better documentation, thank you!