Is your feature request related to a problem? Please describe.
The Google Cloud Ready - BigQuery partnership program requires partners to set a custom UserAgent. This is so Google can correctly attribute usage. However, there is no built in way to set the UserAgent in this library. It's hardcoded to sqlalchemy right now.

Describe the solution you'd like
The create_bigquery_engine method should also accept a user_agent and use that instead of the default sqlalchemy user_agent, if provided.

For now, as a workaround in my code, I'm reimplementing the create_bigquery_engine method myself and setting the client_info directly.