BigQuery: Add support for query parameters

[tswast]

https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs/query#request-body

A query can put placeholders in the query text, which can then be filled in with queryParameters. Depending on what parameterMode is set to, the parameter can be positional ? or names @somename.

I'd expect these to be available on the QueryRequest.

See also: googleapis/google-cloud-python#2551