Add a RESERVED billing event flag for creates of reserved domains

This is an additional signal we can use for the billing/invoicing pipeline that
will be helfpul in targeting invoice rows that may need adjustment.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217524250

javatests/google/registry/model/testdata/schema.txt

@@ -21,6 +21,7 @@ enum google.registry.model.billing.BillingEvent$Flag {
   ANCHOR_TENANT;
   AUTO_RENEW;
   LANDRUSH;
+  RESERVED;
   SUNRISE;
   SYNTHETIC;
 }