title: Include pre-encoded protocol buffer message within outer message
body: <p>Is there a way to create a protocol buffer message in C++ that contains a pre-encoded inner message, without parsing and then re-serializing the inner message?</p>
  
  <p>To clarify, consider the following message definitions:</p>
  
  <pre><code>message Inner {
      required int i = 1;
      // ... more fields ...
  }
  
  message Outer {
      repeated Inner inners = 1;
      // ... more fields ...
  }
  </code></pre>
  
  <p>Suppose you have a collection of 10 byte arrays, each of which contains an encoded version of an Inner. You'd like to create an Outer that contains the 10 Inners. You don't want to hand-encode because Outer has other fields and may itself be included in other messages. Is there a way to get protocol buffers to directly copy the pre-encoded Inner?</p>