Fix voldemort types in public interface

[halzy]

The Fds struct used in ListenFds in the pingora::services::Service trait is not nameable. See https://rust-lang.github.io/rfcs/2145-type-privacy.html#lint-3-voldemort-types-its-reachable-but-i-cant-name-it

This change makes both ListenFds and Fds public.

As a side-effect all of the methods on Fds are now public as well but are not documented.

Fixes: https://github.com/cloudflare/pingora/issues/145