Make KumoProxy a stand-alone package

[tommairs]

KumoProxy is super helpful and easy to use, but deploying it is cumbersome. It would be nice if it were packaged as a separate utility that can be deployed and managed without having to install the full KumoMTA server.

The problem: Right now you need to install the full KumoMTA and then run only the proxy-server binary, which is really the only part you need. Also, it does not actually "install" with a system service. If you want it to run and survive a restart, you need to create your own system service for it.

Ideally, the install should be as easy as sudo apt install kumoproxy