title: Caching supported for callable functions?
body: <p>I was wondering whether it is possible to apply cache-control settings to the response of a callable function. We've used http triggers before and switched to callable functions but cannot find any way to take over the cache-control header.</p>
  
  <p>To be more specific: a http-triggered cloud function allows a cache setting like this:</p>
  
  <pre><code>exports.myFunc = functions.https.onRequest((req, res) =&gt; {
    res.set('Cache-Control', 'public, max-age=600, s-maxage=1200');
    ...
  }
  </code></pre>
  
  <p>When we migrated to a callable function</p>
  
  <pre><code>exports.myFunc = functions.https.onCall((data, context) =&gt; {
    ... 
    /* no res-object for cache-control? */
    ...
  })
  </code></pre>
  
  <p>which gets called in our app as follows:</p>
  
  <pre><code>const myFunc = functions.httpsCallable('myFunc');
  myFunc({params}).then(result =&gt; ...)
  </code></pre>
  
  <p>we couldn't find a way to apply cache settings anywhere.</p>