title: Return list of all dictionary keys
body: <p>Is there a way to return a list of all dictionary keys in the provided input (at all levels)?<br>
  The keys should be ordered by level, for example:   </p>
  
  <pre><code>{
      "A": 1,
      "B": [{
          "B1": 1,
          "B2": 1
      }, {
          "B3": 1,
          "B4": 1
      }],
      "C": {
          "C1": 1,
          "C2": 1
      }
  }
  </code></pre>
  
  <p>I have tried using <code>dict.keys()</code> and <code>dict.items()</code> but did not get the desired output.<br>
  The output should be like this <code>["A", "B", "C", "B1", "B2", "B3", "B4", "C1", "C2"]</code>.<br>
  Any help is appreciated.</p>