fix(gcs): support authorized user ADC credentials

[vladpetric]

Summary

• support Google authorized_user ADC JSON files for GCS by exchanging the refresh token for an access token and passing that token to OpenDAL
• keep non-authorized_user GCS credential files on the existing credential_path path
• report multilevel storage as writable when at least one configured level is writable, so a writable L0 disk cache can still accept writes when a remote L1 is read-only

Motivation

Local Google ADC files created by gcloud auth application-default login use type: authorized_user. The existing OpenDAL credential-path path rejects those files and falls back to metadata-server auth, which fails on normal developer machines.

This lets local sccache users authenticate GCS via their existing user ADC file while preserving the existing service-account behavior.

Testing

• cargo fmt --manifest-path /home/vlad/build/sccache/Cargo.toml --check
• cargo test --manifest-path /home/vlad/build/sccache/Cargo.toml cache::multilevel
• cargo build --manifest-path /home/vlad/build/sccache/Cargo.toml --release
• local smoke test with disk L0 + GCS L1 using an authorized_user ADC file:
  • sccache daemon starts with both levels
  • GCS L1 is readable
  • read-only GCS L1 does not prevent writes to disk L0
  • repeated C compile hits disk L0 on the second invocation

[codecov-commenter]

Codecov Report

❌ Patch coverage is 7.69231% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.96%. Comparing base (5d52f91) to head (f709805).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cache/gcs.rs	0.00%	60 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (5d52f91) and HEAD (f709805). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (5d52f91)	HEAD (f709805)
	14	4

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2733       +/-   ##
===========================================
- Coverage   74.51%   60.96%   -13.55%     
===========================================
  Files          70       69        -1     
  Lines       39652    37944     -1708     
===========================================
- Hits        29546    23133     -6413     
- Misses      10106    14811     +4705     

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[sylvestre]

a bunch of jobs are failing