Proxy is supplied as a pre-built exe with core functionality available via configuration

The original issue #247 got hijacked as a request for docker, so forking this as an issue for creating an exe.

> We've proposed shipping YARP in three formats:
>
> * A library - https://www.nuget.org/packages/Microsoft.ReverseProxy/
> * A template - #77
> * A pre-built application exe (this issue)
>
> The pre-built app should:
>
> * Read from a config file
> * Cover basic scenarios
> * Support a variety of platforms
>
> # Scenarios
> * Config based routes, clusters, & backends
> * Remote route config? E.g. load from a database, service fabric, k8?
> * Dashboards? #123
> * Health checks for the proxy itself.
> * Static files? #187
> * Auth?
> * Multi-instance?
>
> * Data protection (Session Affinity)
> * Single-file exe with linking: #7
> * Config UI? #184
>
> # Config
> The library already supports loading routes from a config file. However, an app will need to expose additional config for the scenarios described above.
>
> # Platforms
> * Windows, Mac, Linux
> * Builds for both runtime dependent and standalone (servicing?)
>
> # Acquisition
> * Microsoft download center?