Add Service Discovery

[monwolf]

Is your feature request related to a problem? Please describe.

I would like to use this proxy server with a service catalog (consul), it would be fine be able to use consul api (HTTP/DNS) discovery to get the backend lists.

Describe the solution you'd like A way to not depend on external LB and avoid extra network hops is by attacking the service directly. Service Catalogs usually provide an API to discover services it would be great if we could take advantage of this technology.

Describe alternatives you've considered

The most versatile alternative is implementing https://www.rfc-editor.org/rfc/rfc2782 for discovering backends