feat(gcp): support explicit bearer token config in parse flow

[siddharthmittal13]

Which issue does this PR close?

Closes #716.

Rationale for this change

GCS already supported static bearer-token authentication through the builder, but there was no clean way to use it through the standard parse_url_opts configuration flow. That forced downstream users to bypass the normal GCS parsing path and manually construct a builder, which risks drifting from the default option-handling behavior.

This change adds explicit bearer-token support to the normal GCS config path so callers can keep using the standard parse/config flow.

What changes are included in this PR?

This PR adds support for configuring GCS with an explicit OAuth bearer token through the standard GCS config parsing path.

It also documents that an explicitly provided bearer token is treated as a static credential and is not refreshed automatically.

In addition, it adds test coverage for bearer-token configuration aliases and the parse-layer GCS flow.

Are there any user-facing changes?

Yes.

Users can now provide an explicit bearer token for GCS using supported config keys such as google_bearer_token and bearer_token while continuing to use the normal parse_url_opts path.

This token is treated as a static credential override and will not refresh automatically.

There are no breaking API changes.