Middleware upgrade (#554)

* Adds root level middleware

* Added todo

* Better way for extensions to be added.

* Bad conflict merge?

fnext/listeners.go

@@ -0,0 +1,31 @@
+package fnext
+
+import (
+	"context"
+
+	"github.com/fnproject/fn/api/models"
+)
+
+// AppListener is an interface used to inject custom code at key points in app lifecycle.
+type AppListener interface {
+	// BeforeAppCreate called right before creating App in the database
+	BeforeAppCreate(ctx context.Context, app *models.App) error
+	// AfterAppCreate called after creating App in the database
+	AfterAppCreate(ctx context.Context, app *models.App) error
+	// BeforeAppUpdate called right before updating App in the database
+	BeforeAppUpdate(ctx context.Context, app *models.App) error
+	// AfterAppUpdate called after updating App in the database
+	AfterAppUpdate(ctx context.Context, app *models.App) error
+	// BeforeAppDelete called right before deleting App in the database
+	BeforeAppDelete(ctx context.Context, app *models.App) error
+	// AfterAppDelete called after deleting App in the database
+	AfterAppDelete(ctx context.Context, app *models.App) error
+}
+
+// CallListener enables callbacks around Call events
+type CallListener interface {
+	// BeforeCall called before a function is executed
+	BeforeCall(ctx context.Context, call *models.Call) error
+	// AfterCall called after a function completes
+	AfterCall(ctx context.Context, call *models.Call) error
+}