add support signing container images by using cosign project

**Describe the problem/challenge you have**
I would like to sign container images while building them through carvel-kbld.

**Describe the solution you'd like**
[cosign](https://github.com/sigstore/cosign) maintained by [sigstore](https://sigstore.dev/) community is a new standard that allows the signature and verification of container images and OCI artifacts.

we (w/@dentrax @erkanzileli) thought that we can add this support for signing images right after built them.

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

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No source files or tests are named. Start by locating kbld's image-building and pushing entry points, then read the linked cosign project documentation; done means container images built through kbld can be signed immediately after they are built.

Written by the indexing model from the issue text.