Fix the representation of arbitrary JSON for input/results

[chendrix]

No description provided.

[charlesdaniels]

Chris and I discussed this previously. We decided that although both an empty schema ({}) or this "oneOf" method could work in that they should be equivalent for valid inputs, there is a possibility that the oneOf style would push some type errors closer to where they happen, e.g. if someone tries to pass a function in as input. That should cause an error up front this way, where otherwise it might instead throw a more obscure error during ser/de or get all the way to OPA and then fail.