storage: support external_account credentials in detectDefaultGoogleAccessID

[tfujiwar]

Is your feature request related to a problem? Please describe.

detectDefaultGoogleAccessID can detect GoogleAccessID when the library is authenticated by GKE workload identity but it cannot when it is authenticated by workload identity federation (e.g. GitHub Actions) because the credential type is external_account.

Describe the solution you'd like

We'd like the function detectDefaultGoogleAccessID to support external_account credentials. If the service_account_impersonation_url value in the credential is available, the function should be able to extract an Email from it just like the case of impersonated_service_account credentials.

Describe alternatives you've considered

We can specify GoogleAccessID explicitly in a user code but we don't want to branch out logics depending on how the library is authenticated.