title: Passing arguments to sql template from airflow operator
body: <p>If I am using a BigQueryOperator with a SQL Template, how could I pass an argument to the SQL?</p>
  
  <p><strong>File</strong>: .sql/query.sql</p>
  
  <pre><code>SELECT * FROM `dataset.{{ task_instance.variable_for_execution }}
  </code></pre>
  
  <p><strong>File</strong>: dag.py</p>
  
  <pre><code>BigQueryOperator(
      task_id='compare_tables',
      sql='./sql/query.sql',
      use_legacy_sql=False,
      dag=dag,
  )
  </code></pre>