BigQueryOption `queryPriority` is specified in types, but not passed onto BigQuery client

After migrating from the previous Google BigQuery data source for Grafana, we are no longer able to specify the Job priority on the data source.

After reading through this repository a bit, it seems like the types for implementing `queryPriority` is already in place (see [here](https://github.com/search?q=repo%3Agrafana%2Fgoogle-bigquery-datasource%20queryPriority&type=code)). However, the `queryPriority` value is not getting passed onto the underlying BigQuery client.

It would be quite beneficial to have this as an option, since we don't want our dashboards to getting prioritised before any of our customer-facing queries.

Trace the existing queryPriority type from the BigQueryOption definition through the query path to the underlying BigQuery client. Confirm how the client receives job options and add coverage if an existing test entry point is found; done means the configured priority reaches BigQuery for dashboard queries.

Written by the indexing model from the issue text.