Add the App Engine service used in the Action definition

Our goal is to be able to address every Action by looking at the class itself, and to make it clearer at a glance what you need to access the Action's endpoint

Currently, we can know from the @Action annotation:
- the endpoint path
- the Method needed
- the authentication level needed

This CL adds the service where the Action is hosted, which also translates to the URL.

NOTE - currently we don't have any Action hosted on multiple services. I don't think we will ever need it (since they do the same thing no matter which service they are on, so why host it twice?), but if we do we'll have to update the code to allow it.

The next step after this is to make sure all the @Parameters are defined on the Action itself, and then we will be able to craft access to the endpoint programatically (or at least verify at run-time we crafted a correct URL)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229375735

java/google/registry/backup/ExportCommitLogDiffAction.java

@@ -60,11 +60,11 @@ import org.joda.time.DateTime;
 
 /** Action that exports the diff between two commit log checkpoints to GCS. */
 @Action(
-  path = ExportCommitLogDiffAction.PATH,
-  method = Action.Method.POST,
-  automaticallyPrintOk = true,
-  auth = Auth.AUTH_INTERNAL_ONLY
-)
+    service = Action.Service.BACKEND,
+    path = ExportCommitLogDiffAction.PATH,
+    method = Action.Method.POST,
+    automaticallyPrintOk = true,
+    auth = Auth.AUTH_INTERNAL_ONLY)
 public final class ExportCommitLogDiffAction implements Runnable {
 
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();