Index feature for proxy upstream organizations

We would like to propose a feature that provides an index of the proxy cache for upstream registries features. 

Example: 

Let's say we configure docker.io as an upstream. 

This feature would contact docker.io and create an index of the current available repositories and tags, this would allow our developers to query what docker.io offers without going outside our secure network and then be able to decide what tags to use and when updates are available.

The primary use the upstream proxy cache feature is in environments where access to the internet is restricted, this means our users and automations who work in such environments would have access to a general index that would let them know what the possibilities are from a tag perspective and also be able to auto deploy and scan new images that are newer than the ones in use.  

The opposite would be that the user has to go outside the environment and find out what tags are present, inconvenient when you are using the Quay as a single source of images.