spec requires support custom fields anywhere in the json :
Implementers who encounter undefined attribute-value pairs in the format must include the data when calculating hashes or verifying signatures and must preserve the data when re-serializing
we should test this. python-tuf has a unrecognized_fields in (all?) it's objects that can be used for this
Mostly the test is interesting if the custom fields are withing "signed" (to ensure hash calculation contains these fields)