Add functionality to install cert for an app to the client

[rockfordlhotka]

We should have some way to add package certificates to the server, and then to install those certificates on the client before trying to install an app.

This is only valid for app testing scenarios, because production apps should be signed with a trusted cert. But for app testing scenarios where the package is signed with a test cert, there should be some way to install that test cert onto the client workstation.