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

java/google/registry/flows/EppXmlTransformer.java

@@ -64,7 +64,8 @@ public class EppXmlTransformer  {
       "smd.xsd",
       "launch.xsd",
       "allocate.xsd",
-      "superuser.xsd");
+      "superuser.xsd",
+      "allocationToken-1.0.xsd");
 
   private static final XmlTransformer INPUT_TRANSFORMER =
       new XmlTransformer(SCHEMAS, EppInput.class);