Add conformance testing

[tetsuo-cpp]

Description

Recently, @woodruffw and I have been working on a conformance testing suite for Sigstore clients. At the moment, there are a handful of tests but we'll be focusing on building out coverage in the coming months. The test suite is exposed via a GitHub Action which can be found here.

The way this works is that we've designed a CLI protocol, which is essentially a set of flags that the client needs to support in order to be conformance tested. My understanding is that sigstore-java doesn't expose a CLI so, in order for this to work, there will need to be a barebones CLI app that supports the flags described in the linked doc.

If you're happy for sigstore-java to be included in this, I'd be happy to make a PR to get things going.

CC: @di