reqsign-google: add built-in provider for static OAuth access token

[Xuanwo]

Problem

reqsign-google currently has:

• DefaultCredentialProvider for ADC/env/well-known/vm metadata
• StaticCredentialProvider for JSON credentials (service account / external account)
• Credential::with_token(Token { ... }) data model

But it does not provide a built-in ProvideCredential implementation for a raw OAuth access token string.

In OpenDAL GCS migration to reqsign v2, this gap requires a local custom provider (e.g. StaticTokenCredentialProvider) only to bridge token config into Signer<Credential>.

Why this matters

• Users often already have a short-lived access token and want to sign requests directly.
• Without a built-in provider, each integrator must re-implement the same tiny adapter.
• It makes migration from legacy integrations noisier than necessary.

Proposal

Add a built-in provider in reqsign-google, for example:

• StaticTokenCredentialProvider::new(access_token)
• Optional helpers for expiration:
  • with_expires_at(Timestamp)
  • with_expires_in(Duration)

Implementation can return Credential::with_token(Token { ... }).

Compatibility

This is additive and backward compatible.

Extra note

A related gap is VM metadata service-account selection (service_account path segment), but this issue focuses only on static token provider.

[Xuanwo]

In fact, StaticCredentialProvider should support both static token and a token path.