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")
 }

_examples/dependency-injection/basic/middleware/main.go

@@ -46,8 +46,11 @@ func newApp() *iris.Application {
 	// a JSON and 200 status code
 	// or 202 status code and empty body
 	// or a 409 status code and "my_error" body.
-	app.UseFunc(middleware)
-	app.HandleFunc(iris.MethodPost, "/{id:int}", handler)
+	di := app.DI()
+	{
+		di.Use(middleware)
+		di.Post("/{id:int}", handler)
+	}
 
 	app.Configure(
 		iris.WithOptimizations, /* optional */