Implement a CDN proxy for the Unreal Engine Marketplace

[hach-que]

We need a CDN proxy that you can install with uet cdn-proxy install and then interactively add Epic Games accounts to it with uet cdn-proxy add-account. The proxy advertises itself on the local network and allows other UET instances to download assets and engines from the Marketplace on-demand. The CDN proxy handles doing the exchange code and refreshing the access token of accounts as needed.

If we wanted to make this a little more fancy, we could instead have uet server install and have a frontend implemented in React that kicks off the account management process. We could then use this server as a starting point for other features down the line like engine Git proxying for faster VFS operations across multiple machines, and incorporating the Io build server frontend and turning UET server into an actual build server with agent connectivity (so we can get around GitLab jank).