Sigstore signature validation and copying

[reegnz]

I've noticed that this project is using skopeo to copy images, that's very cool!
Would be great if this project supported validating and copying sigstore signatures as well.

Skopeo utilizes this config format to validate images: https://github.com/containers/image/blob/main/docs/containers-policy.json.5.md
It can be instructed to look at signatures using this config format: https://github.com/containers/image/blob/main/docs/containers-registries.d.5.md#individual-configuration-sections

Ideally one should be able to pass the necessary configuration files to skopeo by allowing for custom skopeo args to be configured.

[estahn]

@reegnz Thanks. FYI There is currently a PR (#497) to remove the skopeo dependency and handle image pulling natively. This will use the same libraries as skopeo, so it should still be possible to use above.