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
