[Feature] Custom credential provider for authentication to custom (non-ACR) container registries

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

Today, there is no straightforward way to be able to specify and enable images from a custom registry such a user can be authenticated and the pull through from a custom registry will take place.

**Describe the solution you'd like**

I want some way to be able to modify the credential provider, or be able to provide a custom one, which would then be given permissions to from the API server to go and pull images from the intended registry.