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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.82%. Comparing base (4f21ade) to head (9031b0a). Report is 112 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #890   +/-   ##
=======================================
  Coverage   74.82%   74.82%           
=======================================
  Files          31       31           
  Lines        3912     3912           
=======================================
  Hits         2927     2927           
  Misses        850      850           
  Partials      135      135           

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