[WIP] test: simple, lightweight, dependency-free Python proxy server

[jpnurmi]

Replace mitmproxy with a simple, lightweight, dependency-free Python proxy server for integration tests. Supports HTTP proxy, proxy-auth, IPv6 bind, and SOCKS5 cases covered by the test suite.

See also:

• test: stabilize proxy tests #1762
  • https://github.com/getsentry/sentry-native/actions/runs/30931657868/job/92067689146
  • https://github.com/getsentry/sentry-native/actions/runs/30994059904/job/92266775721
  • https://github.com/getsentry/sentry-native/actions/runs/31094910368/job/92594565633
• build(docker): Update Alpine images (3.23 + multi-arch) #1713
  • build(deps): bump mitmproxy from 11.0.0 to 12.2.2 in /tests #1643
  • build(deps): bump msgpack from 1.0.8 to 1.2.1 in /tests #1821
• ci: fix, update, and clean up alpine docker image #1941
  • https://github.com/getsentry/sentry-native/actions/runs/30893425596 (~2h!)

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.03%. Comparing base (219d934) to head (16443e1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1967      +/-   ##
==========================================
+ Coverage   76.01%   76.03%   +0.01%     
==========================================
  Files          94       94              
  Lines       22183    22183              
  Branches     3937     3937              
==========================================
+ Hits        16863    16867       +4     
+ Misses       4436     4430       -6     
- Partials      884      886       +2     

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