new 'Party.DI()' method to return the Party's instance of the new 'APIBuilderDI' and move the DI-relative Router to this new APIBuilderDI

Former-commit-id: 2fb81406c6e3162991c90e0918a3cac1b77c2b54

_examples/dependency-injection/basic/main.go

@@ -22,6 +22,6 @@ func handler(id int, in testInput) testOutput {
 
 func main() {
 	app := iris.New()
-	app.HandleFunc(iris.MethodPost, "/{id:int}", handler)
+	app.DI().Post("/{id:int}", handler)
 	app.Listen(":8080")
 }