Replace ecdsa with cryptography

[JamieMagee]

Replace the vulnerable ecdsa library with the production-grade cryptography library for all ECDSA operations.

The ecdsa package is vulnerable to CVE-2024-23342 and the upstream author explicitly states it should not be used in production:

I don't want people to use this library in production environments. It's a teaching tool, it's a testing tool, it's absolutely not a production grade implementation.

-- tlsfuzzer/python-ecdsa#330

Closes #69

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.31%. Comparing base (195fdcd) to head (405af51).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pythonxbox/common/request_signer.py	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   88.34%   88.31%   -0.03%     
==========================================
  Files          54       54              
  Lines        2857     2868      +11     
==========================================
+ Hits         2524     2533       +9     
- Misses        333      335       +2     

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[tr4nt0r]

Thanks for your contribution. I'm not familiar with this cryptography thing, so your help is very welcome. Just a small change, otherwise it looks good 👍🏼