Automate updating if BigQuery JSON

The BigQuery handwritten libraries use [the discovery JSON file](https://github.com/googleapis/google-cloud-php/blob/main/BigQuery/src/Connection/ServiceDefinition/bigquery-v2.json) 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.