Configure basic configuration values with command flags

[markmandel]

Just for my own edification (and we should probably document this too) - if you don't provide a config file, how do you tell the Quilkin Proxy its new configuration? Don't you at least need to tell it what xDS endpoint to point to?

This does bring up a good point that currently the run command doesn't have a good default state. This was written mostly with manage in mind which does have a good default, and the most important options are available via the command line. I think we should get run into the same state (as a seperate PR).

Using other proxies as prior art (looking at mostly Caddy for the CLI parts, but also Envoy and Traefik), I think we should have provide a few of the important options through the run command that would used if you haven't provided a configuration file. I think if you're invoking run without a config, you essentially want to run quilkin as a simple reverse proxy. So we should have a required --to/--dest flag that you have to provide a socket address to forward packets to. We'd also provide optional --from to change the listening port, and a optional --management-server flag to allow you to dynamically update it (This would also make --to optional). This would also allow us to change the current health code to state the proxies are unhealthy if they currently have no endpoints.

Originally posted by @XAMPPRocky in #555 (comment)