Add AVRO as a valid BigQuery SourceFormat

[objectfox]

Fixes #2627, and brings the function up to date with the BigQuery Documentation.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[objectfox]

I signed it!

[googlebot]

CLAs look good, thanks!

[tseaver]

@jeffk Thanks for the patch! Interesting that we had support for it for the desitnationFormat field for extracts, but not the sourceFormat field for loads: I almost feel like we've been gaslighted again.