feat: report vulnerabilities as GH issues and scan images from registry

[DnPlas]

This PR brings all the changes from KF-6331-dev-branch into main.

• ci: add re-usable workflow for scans from published img and automatic… (ci: add re-usable workflow for scans from published img and automatic… #73)

This commit adds get-published-images-scan-and-report.yaml, a re-usable workflow
that enables repositories to scan images from a public registry (in the case
of the Analytics team it defaults to charmedkubeflow) and reports back
the security vulnerabilities as Github issues.
This workflow is intended to be used on demand (using a workflow dispatch)
and on schedule, as it will be used for continuous testing of the published
images a rock repository generates.

• ci: add workflow to enable automatic vulnerability reports (ci: add workflow to enable automatic vulnerability reports #72)

This re-usable workflow can be used for reporting security vulnerabilities
via Github issues. It takes the issue title, image-name, and issue-labels as
inputs, and in turn:

• edits an existing issue with the same title and updates the vulnerability report
• creates a new issue with the issue-title and adds the vulnerability report in the description

Please NOTE this workflow assumes the existence of vulnerability reports as artefacts
of a workflow run; that is, it expects artefacts named trivy-report- to
be present in the sabe workflow run.

• chore, ci: bump artifact download/upload and checkout actions v3 -> v4 (chore, ci: bump artifact download/upload and checkout actions v3 -> v4 #71)

Bump the version of this actions to be up to date with the latest.

All changes have been tested individually in their respective PRs.