Add base AllocationToken validation logic for domain checks

Next up is adding custom logic so that the results of these checks can be
more meaningful.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181660956

docs/flows.md

@@ -495,6 +495,7 @@ An EPP flow that checks whether domain labels are trademarked.
 *   2002
     *   Command is not allowed in the current registry phase.
     *   Claims checks are not allowed during sunrise.
+    *   Claims checks are not allowed with allocation tokens.
 *   2004
     *   Domain name is under tld which doesn't exist.
 *   2201