[Data] Include ray user-agent in BigQuery client construction

[tswast]

Why are these changes needed?

Including something in the user-agent header indicating requests to BigQuery originated from Ray would help myself and my colleagues on the BigQuery and Vertex AI teams prioritize improvements to the Ray + BigQuery connector. This is something the BigQuery team requires of official partners and by some interpretations of the Google APIs terms of service, specifically "You will not misrepresent or mask either your identity or your API Client's identity when using the APIs or developer accounts.".

Related issue number

For example, similar changes to pandas-gbq (googleapis/python-bigquery-pandas#281) and ibis (ibis-project/ibis#2303) have helped me continue spending a portion of my working hours improving pandas and ibis.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• [N/A] I've included any doc changes needed for https://docs.ray.io/en/master/.
  • [N/A] I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests (python -m pytest -v -s python/ray/data/tests/test_bigquery.py)
  • Release tests
  • This PR is not tested :(