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