This commit adds a method to the `LoadBalancer` interface in the
userspace proxy which allows consumers of the `LoadBalancer` to check if
it thinks a given service has endpoints available.
Moves the userspace code in proxy to a sub-package and adds the
ProxyProvider interface.
This is in preparation for landing an implementation of
https://github.com/GoogleCloudPlatform/kubernetes/issues/3760, which
will mostly be in another sub package for iptables.