title: Add a property to every array element returned in observable
body: <p>I am making an API call using the HttpClient in Angular. It returns a JSON array of User.</p>
  
  <pre><code>users$ = this.httpClient.get&lt;User[]&gt;('api1')
  </code></pre>
  
  <p>I am showing the users (name and email) using the async pipe in the template.</p>
  
  <pre><code>&lt;div *ngFor="let user user of users$ | async"&gt;
  &lt;h1&gt;{{user.name}}&lt;/h1&gt;
  &lt;h2&gt;{{user.email}}&lt;/h2&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Now, for every user, I need to make another (different) API call to get the user's picture URL. How can I efficiently </p>
  
  <ol>
  <li>Save the picture URL in the users$</li>
  <li>Show the picture in the template (img tag)</li>
  </ol>
  
  <p>I don't want to get the images along with the user name and email</p>