support signing of images

**Describe the problem/challenge you have**

i would like to be able to sign images with cosign that are built by kbld. this should work regardless of the builder i use (similar to tagging).

**Describe the solution you'd like**

**Anything else you would like to add:**

- sister issue in vendir for verifying: https://github.com/vmware-tanzu/carvel-vendir/issues/92
- should work well with imgpkg picking up signatures during imgpkg copy
- useful bg content: https://dlorenc.medium.com/policy-and-attestations-89650fd6f4fa

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