Enable private outputs for zkProgram

[garwalsh]

Currently zkProgram takes public and private input and returns public output. It does not currently support private output, a result of executing a proof that is not part of the proof statement itself.

This QoL update would enable private outputs such that zkProgram becomes more flexible and more accurately models how a developer would naturally expect such an API to function.

Example use case: a payment app that works via a claim box. The private output is the link to claim the payment that you only want the zkApp to share with the recipient