Create a new `client.New` to make external API systems easier to
construct.
A new type `client.OptionFunc` allows the client to be
extended with specific configuration.
This also makes a first pass at supporting multiple endpoints properly
by creating a custom grpc resolver.
(Proper load balancing support is still a TODO.)
Fixes#2093
Signed-off-by: Seán C McCord <ulexus@gmail.com>
