Signing built images

Users can sign images produced with `ko publish` using tools like [`cosign`](https://github.com/sigstore/cosign).

For example:

```
$ cosign sign -key cosign.key $(ko publish ./)
```

`ko resolve` produces potentially many images, which makes this a bit harder. You could `ko resolve` then scan the resulting YAML for `ko`-built image references and sign all of those with some bash magic, but 🤮 .

Would it be useful to have a `ko resolve --sign cosign.key` flag that used the provided key to sign all images built during `ko resolve`?

`ko publish --sign cosign.key` could also be a convenience alias for, effectively, `cosign sign $(ko publish)`, which wouldn't require users to have `cosign` installed.

@dlorenc good idea? bad idea?

Start by reviewing the `ko resolve` and `ko publish` entry points and the cosign workflow described in the issue. The scope is not settled: completion would require an agreed signing interface and a clear way to sign all images produced by `ko resolve`, plus the proposed `ko publish` convenience behavior.

Written by the indexing model from the issue text.