etcd: Add world API changes for watching member status

This adds some useful functionality so that anyone with access to the
world API, can learn information about the changing etcd cluster it's
using.

engine/world.go

@@ -20,6 +20,7 @@ package engine
 import (
 	"context"
 
+	"github.com/purpleidea/mgmt/etcd/interfaces"
 	"github.com/purpleidea/mgmt/etcd/scheduler"
 )
 
@@ -51,4 +52,7 @@ type World interface { // TODO: is there a better name for this interface?
 	Scheduler(namespace string, opts ...scheduler.Option) (*scheduler.Result, error)
 
 	Fs(uri string) (Fs, error)
+
+	// WatchMembers returns a channel of changing members in the cluster.
+	WatchMembers(context.Context) (<-chan *interfaces.MembersResult, error)
 }