For the .app discounting logic, we need a new extension which will let
registrars set, clear and query custom flags on a domain. Hopefully
this will be reusable for other custom TLDs later. This CL adds the
XSD, the associated classes for marshalling and unmarshalling, and some
marshalling tests, and links the classes into the system-wide extension
lists.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128178999
