Add registry scanner module

[ishitasequeira]

Image Updater provides some functionality that can be reused by other projects, most notably the feature to inspect OCI and Docker registries' contents and pick an image based on some constraints.

This PR starts the initial setup of a new module named registry-scanner to separate out the functionality of registry scanning and fetching images into a library. This allows users to pull in just the registry scanning functionality and not pull in the entire image updater for use.

Future Tasks:

• Replicate part of pkg/ folders to registry-scanner baring the argo-cd functionality
• Integrate registry-scanner with argocd-image-updater
• Update github CI to execute registry-scanner checks only when updating files in the module
• Releasing registry-scanner as part of release process.