Add domain check and update registration type extensions

This completes the command extensions for the regType 0.2 extension.
Up next will be the response extensions.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123322887

javatests/google/registry/flows/domain/DomainUpdateFlowTest.java

@@ -1108,4 +1108,12 @@ public class DomainUpdateFlowTest extends ResourceFlowTestCase<DomainUpdateFlow,
             .build());
     doSuccessfulTest();
   }
+
+  @Test
+  public void testSuccess_regTypeExtensionValidates() throws Exception {
+    setEppInput("domain_update_regtype.xml");
+    persistReferencedEntities();
+    persistDomain();
+    doSuccessfulTest();
+  }
 }