added the Middleware interface

the middleware interface helps with middleware that requires to have
backgroundtasks or open connections.

middleware/middleware.go

@@ -47,3 +47,9 @@ func RequireUser(c *gin.Context) {
 		return
 	}
 }
+
+// Middleware is an type to Save data between executions and to provide help at the teardown.
+type Middleware interface {
+	Gin(*gin.Context)
+	Teardown()
+}