Support for JSON query parameters

[maingoh]

It is currently not possible to escape json parameters in a query like this:

job_config = bigquery.QueryJobConfig(
    config.query_parameters=[
        bigquery.JsonQueryParameter("data", {"foo": "bar"})
    ]
)
stmt = 'UPDATE FROM my_table SET data=@data';
query_job = client.query(stmt, job_config=job_config)

It seem quite important to avoid SQL injections.