Permissions for env variables

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [x] Are you an Up Pro subscriber?

## Description
Up Pro feature request. Would it be possible to set access permissions on env variables of certain stages for up team members? For example, we'd like to limit access to production credentials!