Support BigQuery oauth using a bearer token

[drewbanin]

Describe the feature

dbt Core should support BigQuery targets configured directly with a Bearer token for OAuth authentication. This will enable OAuth authentication without requiring manual gcloud authorization steps to log into GCP.

This feature should look similar to the Snowflake OAuth flow implemented in #2050. Let's create a new auth method called bearer which accepts the following fields:

• client_id
• client_secret
• refresh_token
• token_uri

Given these credentials, dbt should be able to mint new access tokens that are used to authorize BigQuery queries.

Who will this benefit?

Services that invoke dbt on behalf of a user (ie. where Service Account auth isn't desirable). We plan on leveraging this functionality in dbt Cloud to support OAuth dev creds for BigQuery credentials, but anyone should be able to use this API too.