[Feature] Support google.auth.credentials.AnonymousCredentials for bigquery enabling to unit test models.

[shrivastava-ankur]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt-bigquery functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Support google.auth.credentials.AnonymousCredentials for bigquery enabling to unit test models in isolation locally.

• use of AnonymousCredentials
• Ability to pass api_endpoint as client_options to client instantiation

Describe alternatives you've considered

No available options to connect to bigquery-emulator instance for isolated testing.

Who will this benefit?

All the dbt developers using bigquery will be benefitted, as this change would allow them to test the models in isolation.

Are you interested in contributing this feature?

Yes, I am ready with the changes.

Anything else?

No response