Add ready property to ReviewResult

This is particularly needed for transitioning PolyGerrit to use this new
API. We can use this property to determine immediately upon receiving a
response to posting a review whether we need to fall back to a second
API call to start review.

Bug: Issue 6760
Change-Id: Ib0ef5f5a0d454de4cd4b9496ab1cf81dba729a99

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/ReviewResult.java

@@ -31,6 +31,11 @@ public class ReviewResult {
    */
   @Nullable public Map<String, AddReviewerResult> reviewers;
 
+  /**
+   * Boolean indicating whether the change was moved out of WIP by this review. Either true or null.
+   */
+  @Nullable public Boolean ready;
+
   /** Error message for non-200 responses. */
   @Nullable public String error;
 }