First-class UX for Helm charts

Customer Problem: As a Quay users I want to be able to manage my Helm charts with Quay. The simple push/pull experience is not enough. I need to know which versions of the Helm chart are currently stored and I need to verify that the metadata for each of those version is correct, so it can be shown to my potential consumers before they pull the cart.

Goal: Support for Helm v3 charts

Problem:

• as of today customers can't use Quay to store their Helm v3 charts. The existing Quay AppRegistry has been marked as deprecated and we might want to implement a new method to store artefacts including Helm charts more aligned to the upcoming OCI distribution spec.

Why is this important: 

• AppRegistry deprecated and doesn't support Helm v3
• Helm v3 support recently introduced to OpenShift (Tech Preview)

Dependencies (internal and external):

• OCI distribution spec

Prioritized epics + deliverables (in scope / not in scope):

• As a user I can push and pull Helm charts as I can do with images
• As a user I can view and manage versions of Helm charts stored in Quay
• As a user I can see the corresponding image metdata referenced by the Helm chart (link to repo/tag view including Clair scan results)
• As a user I can use repo mirroring for Helm charts as I can use it for images

Estimate (XS, S, M, L, XL, XXL):  TBD

Previous Work: 

• Quay AppRegistry

Open questions:

• Do we want / need to allow version management via Time Machine as used for images?
• Do we want to support other / similar artefacts as well such as OCP deployment templates, CNAB, etc?