Package authors can attach release notes when releasing with `kctrl`

**Describe the problem/challenge you have**

Release notes are important. Keeping them close to released artifacts is a good idea. `Package.spec.releaseNotes` is a holder of free-form release notes. This enables high cohesion of release notes and the package artifact.

Unlike the contents of `PackageBuild` release notes are will be different for every (pre)release.

However, when releasing packages with `kctrl package release` there is no easy way to attach release notes. Granted, one can template `package-resources.yml` with `Package.spec.releaseNotes` but that can be awkward.

**Describe the solution you'd like**

When I release a package with `kctrl package release` then I want to provide release notes for the version I am releasing.

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

It is conceivable to provide release notes either as a literal string or by referring to a file. However, if there was a hypothetical `--release-notes` flag, then `--release-notes "$(cat my-release-notes.md)"` would be a working as well. Eventually, a `--release-notes-file` could be considered.

If no release notes are provided, then the `Package` should not contain release notes, i.e. there should be no `Package.spec.releaseNotes: ""`.

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