support OCI image index media-type on manifests endpoint

[rchincha]

Is your feature request related to a problem? Please describe.

Currently, zot only allows uploading individual images. It is also possible to upload multiple images in the form of a OCI image index, for example, in multi-arch image upload.

• [ ] add push/pull support
• [ ] add unit tests
• [ ] resolve delete support
• [ ] resolve tags listing, catalog
• [ ] add ecosystem tool verification

[thesayyn]

@rchincha I'd be happy to help with this if you need any.

[rchincha]

@thesayyn, appreciate the offer to help. We already have a PR which should be merged soon. Do you want to look over https://github.com/project-zot/zot/labels/good%20first%20issue for any unassigned issues that might interest you?