Automate updating if BigQuery JSON

[bshaffer]

The BigQuery handwritten libraries use the discovery JSON file to make calls to the BigQuery REST API. However, this is manually updated, and as a result usually does not happen frequently enough.

We can create a GitHub Actions job to submit a PR which keep this up-to-date. This would be a huge help. This may be something we could do using owlbot.py, even though no code is copied, in order to take advantage of owlbot PR submission.