[FEATURE] Add support for signing and pushing signatures with sigstore

[travier]

Is your feature request related to a problem? Please describe.

I can not use this action to sign a container with sigstore when pushing it.

Describe the solution you'd like

Add an option to the GitHub Action to be able to pass a sigstore private key to use when pushing to sign the containers.

Describe alternatives you've considered

The cosign action does not let me push and sign a container in a single atomic step.

Additional context

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/building_running_and_managing_containers/assembly_signing-container-images_building-running-and-managing-containers#proc_signing-container-images-with-sigstore-signatures-using-a-private-key_assembly_signing-container-images

[travier]

CC @vrothberg