extension: GET /v2/_zot/ext/unpack to stream the unpacked image

[rchincha]

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

Some 3rd party services or tools may need remote access to the unpacked image. Can zot help with this?

Describe the solution you'd like

Expose an extension that unpacks the full image.

Describe alternatives you've considered

One could also take the help of 3rd party client tools like skopeo and umoci to do this step.