More POD arg filtering for GPCPCD prove screen

[ax0]

Summary of this PR:

• Additional filtering to GPCPCD's input POD validator, viz.
  • List membership checks for entry values, and
  • Prescribed value checks for entries and signers' public keys.
• Tests

Notably absent are checks for tuples. This will require further changes to the frontend, which this PR does not address.

Marking as draft pending changes to the consumer client.