title: External_query with dynamic query - Error: Connection argument in EXTERNAL_QUERY must be a literal string or query parameter
body: <p>I'm trying to call external_query with dynamically created query string in BigQuery Web UI.</p>
  
  <pre><code>DECLARE extquery STRING;
  SET extquery = "SELECT * FROM mytable;";
  SELECT * FROM EXTERNAL_QUERY("my-external-conn", extquery);
  </code></pre>
  
  <p>This script gives an error</p>
  
  <pre><code>Query error: Invalid table-valued function EXTERNAL_QUERY
  Connection argument in EXTERNAL_QUERY must be a literal string or query parameter
  </code></pre>
  
  <p>Is it impossible to do in BigQuery?
  What is the right way to pass query as a parameter?</p>