fix: honor GCS skip_signature during uploads

[RanaPriyansh]

Which issue does this PR close?

Closes #520.

Rationale for this change

GCS upload requests fetched credentials and added a Bearer header even when with_skip_signature(true) was set. Request::send bypassed the existing skip-aware credential helper.

What changes are included in this PR?

Route upload requests through GoogleCloudStorageConfig::get_credential() and the optional Bearer-auth helper. Add a mock-server regression that checks credential calls and Authorization headers with signing enabled and disabled.

Are there any user-facing changes?

GCS uploads with skip_signature=true no longer fetch credentials at request time or send an Authorization header. The public API is unchanged.

Validation:

• The new regression failed on the upstream base: the skipped request called the credential provider, raising the count from 1 to 2.
• The focused regression passed after the repair.
• cargo test --features=aws,azure,gcp,http passed: 264 unit tests, 3 integration tests, 1 HTTP test, and 57 doctests. Six unit tests and two doctests were ignored.
• Strict Clippy passed for --features gcp and --no-default-features --features gcp-base. Formatting and diff checks passed.
• cargo test --features gcp had one unrelated doctest failure: a doctest imports Azure when the Azure feature is disabled. The doctest passed with the repository CI feature set above.

Live GCS and custom-connector behavior were not tested. Builder-time credential-file parsing is unchanged.

Codex used.