Create a very basic library/CLI for loading PollyPackage/validating pops

[sdboyer]

Starting with the Go CUE SDK is pretty intimidating. I've done it a few times already. To help bridge that gap for others/start bridging the gap between polly and other existing toolchains, i'll make a very simple library for interacting with pops, and a corresponding CLI that will validate a pop against the PollyPackage schema.

First pass at it is gonna be really, really sloppy. Lots of fodder for issues and requests for improvement :)

[sdboyer]

Various other things have come up, but i'm finally getting back to this this week :)