Add simple registration type command extension for domain creates

Note that it doesn't do anything yet beyond basic XML validation
because the default registry system doesn't use registration types,
but this serves as a template for the other domain commands using
registration types and provides a method that TLDs implementing custom
logic can use.  This also explicitly doesn't yet handle the response
extensions.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123245388

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

@@ -1274,4 +1274,11 @@ public class DomainCreateFlowTest extends ResourceFlowTestCase<DomainCreateFlow,
     persistContactsAndHosts();
     doSuccessfulTest();
   }
+
+  @Test
+  public void testSuccess_regTypeExtensionValidates() throws Exception {
+    setEppInput("domain_create_regtype.xml");
+    persistContactsAndHosts();
+    doSuccessfulTest("tld", "domain_create_response.xml");
+  }
 }