Support different output artifacts from Chains

[lcarva]

Some of the policies in this repo look at the IMAGE_URL and IMAGE_DIGEST results included in the SLSA Provenance attestation generated by Tekton Chains. This is often used to answer the question "which Task produced the image being validated?"

However, those are not the only results that Chains understands.

Acceptance Criteria

• Policy rules understand all output artifacts supported by Tekton Chains.