Allow Custom User Agent in `to_gbq()`

[b-schmeling]

Problem

As part of the BigQuery partnership program, my organization was requested to send our own user-agent string with our requests.

We use pandas to_gbq() in some parts of our code. However, there is no way for us to send along our own custom user agent string, as pandas sets that automatically here to "pandas-{}".format(pandas.__version__)

Solution I would like to be able to pass a user agent string or client info object to DataFrame.to_gbq() to be prepended to the user agent string used by pandas.

Alternatives Monkey patching

Additional context