Mint: Add slowapi

[callebtc]

This pull request adds the slowapi library to the project. The slowapi library is used for rate limiting requests to the mint API.

[codecov]

Codecov Report

Attention: Patch coverage is 14.77273% with 75 lines in your changes are missing coverage. Please review.

Project coverage is 63.00%. Comparing base (b288a6d) to head (7a94b1b).

Files	Patch %	Lines
cashu/mint/middleware.py	0.00%	27 Missing ⚠️
cashu/mint/limit.py	0.00%	18 Missing ⚠️
cashu/mint/router.py	0.00%	14 Missing ⚠️
cashu/mint/app.py	0.00%	9 Missing ⚠️
cashu/mint/router_deprecated.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #481      +/-   ##
==========================================
- Coverage   68.38%   63.00%   -5.38%     
==========================================
  Files          74       76       +2     
  Lines        6760     6815      +55     
==========================================
- Hits         4623     4294     -329     
- Misses       2137     2521     +384     

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