Investigate: Container registry allowlisting

In response to the following customer feedback:

• Ability to configure the imagePullPolicy of the init container image (alpine/git) of the workspace pod 
• Support workspace images to be pulled from private registries

A possible solution is to provide the users with the ability to allowlist container registries so that all the images in the devfile should only use that container registry. This could be more widely needed for enterprise users.