feat: Add axum impl for dist server

[Xuanwo]

Close #2234

This PR adds an Axum implementation for the dist server. We introduced this as a new feature called dist-server-axum with no functional changes to the existing server. This allows our community to test the new server as an experimental option.

All behavior remains identical, and all configurations are unchanged. You can run this server exactly as before by simply replacing the binary.

We welcome your feedback and trials. This is just the first step toward modernizing sccache, with the goal of making it a fully async, modular, and easier-to-maintain project.

---

This PR was primarily authored with Amp using Claude-Sonnet-4.5 and then hand-reviewed by me. I AM responsible for every change made in this PR. I aimed to keep it aligned with our goals, though I may have missed minor issues. Please flag anything that feels off, I'll fix it quickly.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 29.42416% with 1005 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.72%. Comparing base (1e2f32a) to head (8a5dd2f).

Files with missing lines	Patch %	Lines
src/dist/http_axum/scheduler.rs	0.00%	332 Missing ⚠️
src/dist/http_axum/server.rs	0.00%	320 Missing ⚠️
src/dist/http_axum/streaming.rs	0.00%	82 Missing ⚠️
src/dist/http_axum/extractors.rs	21.95%	64 Missing ⚠️
src/dist/http_axum/auth.rs	41.17%	60 Missing ⚠️
src/bin/sccache-dist/main.rs	0.00%	45 Missing ⚠️
tests/dist_axum_compat.rs	78.35%	42 Missing ⚠️
src/dist/http_axum/tls.rs	73.48%	35 Missing ⚠️
src/dist/http_axum/tests.rs	81.48%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2464      +/-   ##
==========================================
- Coverage   71.22%   69.72%   -1.50%     
==========================================
  Files          64       72       +8     
  Lines       35221    36625    +1404     
==========================================
+ Hits        25087    25538     +451     
- Misses      10134    11087     +953     

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[sylvestre]

did you evaluate any of the wins here ? thanks