Add support for impersonating a service account with BigQuery

[bbhoss]

resolves #2672

Description

See #2672. Opening up this draft as I hope to get some early feedback. Tests and docs are the main TODOs, I still need to figure out how the testing works.

Checklist

• I have signed the CLA
• 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
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

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[bbhoss]

I added a test but it doesn't seem very useful. Not really sure the existing tests are setup in a way to easily test this without major mocking, I don't see any other authentication tests. Let me know what else is needed.

[jtcohen6]

@bbhoss You're right, in general we don't have as much testing for non-default authentication methods (Snowflake SSO being the big exception). In this specific case, since CircleCI runs our integration tests against BigQuery using a service account (and it doesn't sound like they support oauth), I don't know if we have a better option.

I'm still going to test this locally by oauth'ing into our sandboxed BigQuery account and impersonating Circle's service account.

[bbhoss]

@jtcohen6 added docs here in case you need a walkthrough for testing locally. Let me know if there's anything else I can do :)

[jtcohen6]

LGTM! Thanks for writing up those docs.

We don't have rigorous tests for this, but the code change is quite self-contained. As long as it worked for you in your project and for me in a dummy project, I feel good about merging this in (tomorrow)

[jtcohen6]

Thanks for contributing!