Separate port and host in configuration

[paulpach]

Many cloud providers have an environment variable PORT.
an application is supposed to listen in that port.

Currently, the yaml for listen address looks like this:

backend:
  listen: 0.0.0.0:7000

the problem is that I can't use the port environment variable there.

This PR changes it to this:

backend:
  listen:
    host: 0.0.0.0
    port: 7000

if I want to use the PORT environment variable, now I can do it like this:

backend:
  listen:
    host: 0.0.0.0
    port:
      $secret:
         env: PORT

[Rugvip]

Thanks! 🎉

As mentioned on Discord, I think it would be best if we could keep the existing way of specifying the listen address too. Letting backend.listen be either a string or an object. You can check it with typeof config.get('backend.listen') === 'string'

[Rugvip]

LGTM! 👍