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/DomainCreateFlowTest.java

@@ -380,6 +380,14 @@ public class DomainCreateFlowTest extends ResourceFlowTestCase<DomainCreateFlow,
     doSuccessfulTest();
   }
 
+  @Test
+  public void testSuccess_allocationToken_isIgnored() throws Exception {
+    // TODO(b/70628322): Change this test to fail on invalid allocationToken.
+    setEppInput("domain_create_allocationtoken.xml");
+    persistContactsAndHosts();
+    doSuccessfulTest();
+  }
+
   @Test
   public void testSuccess_multipartTld() throws Exception {
     createTld("foo.tld");