FEATURE: Add support for google.auth.credentials.AnonymousCredentials

[shrivastava-ankur]

resolves #
#1377

Problem

Currently there is no way to run dbt-bigquery locally using big-query emulator.

Solution

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

• introduced new connection method anonymous to use of AnonymousCredentials
• Ability client_options (specifically api_endpoint to connect to local instance) to client instantiation

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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