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

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

java/google/registry/flows/domain/DomainDeleteFlow.java

@@ -100,6 +100,7 @@ import org.joda.time.Duration;
 /**
  * An EPP flow that deletes a domain.
  *
+ * @error {@link google.registry.flows.EppException.UnimplementedExtensionException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceDoesNotExistException}
  * @error {@link google.registry.flows.ResourceFlowUtils.ResourceNotOwnedException}
  * @error {@link google.registry.flows.exceptions.OnlyToolCanPassMetadataException}