Add SignaturePullSecrets e2e test

[hectorj2f]

Description

Add a signature pull secrets e2e test that uses a private registry with specific credentials to verify the usage of the signaturePullSecrets in source.

[mattmoor]

We should try adding a leg that spins up KinD with these options: https://github.com/chainguard-dev/actions/blob/main/setup-kind/action.yaml#L46

The above action logic is probably not super well tested, but it was based on https://github.com/mattmoor/mink/blob/b9148a39b2d8bbc69ca9aaf5e89a7613c0b179d8/hack/setup-kind.sh#L230-L251 which worked reasonably at some point 😅

... and ensure that we can fetch:

1. private image/index sig/att/sbom (with imagePullSecrets and signaturePullSecrets).
2. private image/index configs (re: fetchConfigFile)

[hectorj2f]

Thanks for the suggestion. I wasn't aware of that parameter for kind. I was working on an e2e test that creates an additional private registry container in parallel to the current one.