GitHub Action for Conftest Setup

[peteroneilljr]

Original Requestor: anthonyabarbieri
Original Thread: https://openpolicyagent.slack.com/archives/CLE8VEKGT/p1650889161.145719

Message:

Hey Folks, I put together a setup-conftest action this weekend based off the setup-opa action in the OPA organization. It downloads the conftest binary and adds it to the path for use in other steps: https://github.com/princespaghetti/setup-conftest