engine.Installer: a standard interface for "installable" services

Installer is a standard interface for objects which can "install"
themselves an engine by registering handlers.

This can be used as an entrypoint for external plugins etc.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

engine/engine.go

@@ -10,6 +10,13 @@ import (
 	"strings"
 )
 
+// Installer is a standard interface for objects which can "install" themselves
+// on an engine by registering handlers.
+// This can be used as an entrypoint for external plugins etc.
+type Installer interface {
+	Install(*Engine) error
+}
+
 type Handler func(*Job) Status
 
 var globalHandlers map[string]Handler