title: Extending Protobuf Messages
body: <p>I have many different schemas, however there are a set of fields which every schema contains. I was wondering if there was a way to have a different schema extend a parent schema and inherit its fields. For example this is what I want:</p>
  
  <pre><code>message Parent {
      required string common1 = 0;
      optional string common2 = 1;
  }
  
  message Child1 { // can we extend the Parent?
      // I want common1, common2 to be fields here
      required int c1 = 2;
      required string c2 = 3;
  }
  
  message Child2 { // can we extend Parent?
      // I want common1, common2 to be fields here
      repeated int c3 = 2;
      repeated string c4 = 3;
  }
  </code></pre>
  
  <p>Such that Child1 and Child2 also contain the fields common1 and common2 (and potentially more) from Parent.</p>
  
  <p>Is this possible and if so how?</p>