Stand-up a scoped registry server for upm

[StephenHodgson]

Overview

With the xrtk packages having the ability to be loaded via git, we also need to stand up a server to get the right scoped registries.

Requirements

• XRTK-Core package should be able to add the scoped registries for the toolkit to the project's manifest.json
• Developers should be able to search and download packages using the scoped registry in the editor's package manager.