First draft of modifying RunnerListener to CallListener to get it closer to the action (and named better).

api/server/listeners.go

@@ -0,0 +1,8 @@
+package server
+
+import "github.com/fnproject/fn/api/extenders"
+
+// AddCallListener adds a listener that will be fired before and after a function is executed.
+func (s *Server) AddCallListener(listener extenders.CallListener) {
+	s.Agent.AddCallListener(listener)
+}