Add a SYNTHETIC flag for BillingEvent

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119559193

java/com/google/domain/registry/model/billing/BillingEvent.java

@@ -70,7 +70,12 @@ public abstract class BillingEvent extends ImmutableObject
     ANCHOR_TENANT,
     AUTO_RENEW,
     LANDRUSH,
-    SUNRISE
+    SUNRISE,
+    /**
+     * This flag will be added to any {@link OneTime} events that are created via, e.g., an
+     * automated process to expand {@link Recurring} events.
+     */
+    SYNTHETIC
   }
 
   /** Entity id. */