Add conformance testing

[woodruffw]

Hello sigstore-rs devs!

Description

This is a tracking issue for conformance testing between this client implementation and other Sigstore clients, similar to https://github.com/sigstore/sigstore-java/issues/236 and https://github.com/sigstore/sigstore-python/issues/297.

The rough idea here:

• Implement a small "driver" for this Sigstore client, one that adheres to the CLI protocol for the conformance suite
• Add a new conformance.yml workflow that applies the trailofbits/sigstore-conformance action via the driver

@tetsuo-cpp and I are happy to take a stab at the work here, whenever you feel it's appropriate!

cc @di