BigQuery connector

Google BigQuery, wrapped in its native client's PEP 249 (DB-API) adapter so it shares the same base as the other SQL connectors.

# sources.yaml
warehouse:
  type: bigquery
  project: my-gcp-project
  location: EU                       # optional
  credentials_file: service-account.json   # path relative to the project

Key	Purpose
`project`	GCP project id.
`location`	Dataset location (optional).
`credentials_file`	Service-account JSON (or `credentials_path`).

If credentials_file is omitted, Application Default Credentials are used. Install: uv add 'dashdown-md[bigquery]'.