Add ec.oci.image_index rego function

[lcarva]

EC provides the ec.oci.image_manifest rego function which returns an Image Manifest given an image reference.

If the provided image reference refers to an Image Index, then ec.oci.image_manifest picks the correct Image Manifest and returns that. This is expected.

However, there are cases where we need to know if a certain image reference is for an Image Manifest or an Image Index. This is currently not possible due to the auto-resolution behavior mentioned above.

Acceptance Criteria

• EC provides a new rego function, ec.oci.image_index, that returns an Image Index. If the image reference provided is not an Image Index, log a warning and return no value.

[arewm]

The OCI descriptor function that I added can be used in this case as well: #2220

After fetching the descriptor, you can check the mediaType.