feat: add support for github container registry

[OAyomide]

Add support for the github container registry

This PR adds support for the github container registry and works quite similarly to the docker implementation but also quite different and important implementation differences.

Issues/Limitations

• GHCR registry api is quite opaque and seems to not exactly fully conform to the OCI spec. Majorly, ImageIndex is not returned. A single ImageManifest is returned though. It also does not seem to not return an auth token if the image is being fetched via digest (eg super-linter@sha256:83d590890328d42191c974cb1dceba0d3ad4a994a1e306e73d50d64218bd23ab).

The consequence of this is that:

1. Ghcr images are pulled reliably only with the latest tag.
2. Implementation wise, fetch_index is redundant for Github registry (but required for docker and presumably the not-yet-implemented Google container registry), making the oci trait a little inconsistent (cos a method is more specific to a registry)

• Private images: unable to a find reliable private container to test with. So the assumption is that if the public images work, the private should.

Testing

You can use the public super-linter image to test.

something like: msb pull ghcr.io github/super-linter

Post-merge/Implementation to-do

• Test with private images
• Add implementation for private image (repo) auth. See here for information about Github Container auth