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/DomainCheckFlowTest.java

@@ -111,6 +111,17 @@ public class DomainCheckFlowTest
         create(true, "example3.tld", null));
   }
 
+  @Test
+  public void testSuccess_oneExists_allocationTokenDoesNothing() throws Exception {
+    // TODO(b/70628322): Change this test to fail on invalid allocationToken.
+    setEppInput("domain_check_allocationtoken.xml");
+    persistActiveDomain("example1.tld");
+    doCheckTest(
+        create(false, "example1.tld", "In use"),
+        create(true, "example2.tld", null),
+        create(true, "example3.tld", null));
+  }
+
   @Test
   public void testSuccess_oneReserved() throws Exception {
     setEppInput("domain_check_one_tld_reserved.xml");