title: Custom pipe must be async
body: <p>I have this pipe that's already used throughout the app and it's not to be updated with some async logic. So, the <code>PipeTransform</code> interface expects a synchronous <code>transform</code> method in my component.</p>
  
  <p>The only option that worked for me is adding a <code>| async</code> pipe in all my views. Not the best as my custom pipe would always need that downstream <code>async</code> pipe.</p>
  
  <p>Is there a way to create custom <code>PipeTransform</code> implementations whose <code>transform</code> method can handle async code?</p>
  
  <p>I'm trying to get something like the following to work, with no success so far:</p>
  
  <pre><code>transform(value, args) {
      const source$ = of("abc");
      const response = await source$.pipe(first()).toPromise();
      return response;
    }
  </code></pre>