Add ability to mark pkg repository as "view-only" so that packages are not available to be installed

**Describe the problem/challenge you have**

It might be useful for user to add a package repository to view its contents but not have kapp-controller use it for installation (as part of auto updates).

this is also useful for when you would like to add a pkg repo to a mgmt cluster to view contents, but do not want those packages be available for install on the mgmt cluster itself.