Add and wire up the AllocationTokenExtension XML class

The extension itself isn't used for anything yet; allocation tokens can be
passed for domain creates and checks but are ignored if present. This will
be changed in a subsequent CL that adds AllocationToken entities and related
logic. Usage of this extension in any other EPP flow will throw an
UnsupportedExtensionException.

The relevant spec is https://tools.ietf.org/html/draft-gould-allocation-token-04

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181343188

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

@@ -54,6 +54,7 @@ import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableSortedMap;
 import com.googlecode.objectify.Key;
 import google.registry.flows.EppException;
+import google.registry.flows.EppException.UnimplementedExtensionException;
 import google.registry.flows.EppRequestSource;
 import google.registry.flows.ResourceFlowTestCase;
 import google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException;
@@ -1077,4 +1078,11 @@ public class DomainDeleteFlowTest extends ResourceFlowTestCase<DomainDeleteFlow,
         CommitMode.LIVE, UserPrivileges.SUPERUSER, loadFile("domain_delete_response.xml"));
     assertThat(reloadResourceByForeignKey()).isNull();
   }
+
+  @Test
+  public void testFailure_allocationTokenNotSupportedOnDelete() throws Exception {
+    setEppInput("domain_delete_allocationtoken.xml");
+    EppException thrown = expectThrows(UnimplementedExtensionException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+  }
 }