Bigquery plugin to support custom source

[trangkramp]

Description & motivation

At my company, we have a custom source macro to override the builtins source macro of dbt to limit the data processing (this is the era before --sample flag and we have more logic there than the --sample flag can provide).

We also would like to make use of dbt-external-tables to onboard/manage data from Google Sheets to Google BigQuery via dbt. However, our custom source macro returns a string, hence, we got this error Encountered an error while running operation: Database Error 'str' object has no attribute 'database' when running command:

dbt run-operation stage_external_sources

Hence, in this PR, I would like to explicitly declare the source function from builtins instead of following the macro namespace's search order defined in the dbt_project.yml, so that dbt-external-tables package will make use of the builtins source macro instead of custom source macro.

Checklist

• I have verified that these changes work locally

[trangkramp]

@jeremyyeo can you help me take a look at the PR please? I got an error on the run_tests for snowflake, which is related to the setup of Snowflake I think

Database Error
    250001 (08001): Failed to connect to DB: ***.snowflakecomputing.com:443. Incorrect username or password was specified.

[github-actions]

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