Add basic AllocationToken entity and tests

Logic actually using this entity to follow in subsequent CLs introducing
the command to generate/save these entities as well as the flow logic for
considering them during domain EPP operations.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181342600

java/google/registry/model/EntityClasses.java

@@ -23,6 +23,7 @@ import google.registry.model.common.Cursor;
 import google.registry.model.common.EntityGroupRoot;
 import google.registry.model.common.GaeUserIdConverter;
 import google.registry.model.contact.ContactResource;
+import google.registry.model.domain.AllocationToken;
 import google.registry.model.domain.DomainApplication;
 import google.registry.model.domain.DomainBase;
 import google.registry.model.domain.DomainResource;
@@ -62,6 +63,7 @@ public final class EntityClasses {
   @SuppressWarnings("unchecked") // varargs
   public static final ImmutableSet<Class<? extends ImmutableObject>> ALL_CLASSES =
       ImmutableSet.of(
+          AllocationToken.class,
           BillingEvent.Cancellation.class,
           BillingEvent.Modification.class,
           BillingEvent.OneTime.class,