Let cherrypick API optionally preserve reviewers/ccs

I don't plan to change the default, or the action performed
by the cherry-pick button in the UI, but adding this option
to the API will allow API customers (like Chromium's "Reland"
button) to carry forward some metadata from the original
change to the newly-created one.

Change-Id: I1d149d2758b825135460564dbf8038f4d7d52f0f

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

@@ -26,4 +26,6 @@ public class CherryPickInput {
 
   public NotifyHandling notify = NotifyHandling.NONE;
   public Map<RecipientType, NotifyInfo> notifyDetails;
+
+  public boolean keepReviewers;
 }