[engine][ast]Implement Break Statement

[summer-ji-eng]

To support write break statement:
Use case in composing sample code like:

   * try (EchoClient echoClient = EchoClient.create()) {
   *   PagedExpandRequest request = PagedExpandRequest.newBuilder().build();
   *   while (true) {
   *     PagedExpandResponse response = EchoClient.pagedExpandCallable().call(request);
   *     for (PagedExpandResponse element : response.getResponsesList()) {
   *       // doThingsWith(element);
   *     }
   *     String nextPageToken = response.getNextPageToken();
   *     if (!String.isNullOrEmpty(nextPageToken)) {
   *       request = PagedExpandRequest.toBuilder().setPageToken(nextPageToken).build();
   *     } else {
   *       break;
   *     }
   *   }
   * }

[miraleung]

Could we please update the sample sniippet in the description as well?