title: In a proto3 message is there a way to mark a field as not required for requests and required for response?
body: <p>I have the following proto3 message structure:</p>
  <pre><code>message BaseBuildContent {
    string locale = 1;
    string buildVersion = 2;
    string buildLabel = 3;
    google.protobuf.Timestamp createTime = 4;
  }
  </code></pre>
  <p>I am using the &quot;same&quot; structure for some requests and responses on my app. What I want to achieve is to mark somehow (if possible) the createTime  field as not required, in case we are talking about a request object, and required in case we are taking about a response object.</p>
  <p>Is it possible to do this without creating a separate message ?</p>
  <p>Thanks</p>