add create user action class

src/Application/Actions/User/CreateUserAction.php

+<?php
+declare(strict_types=1);
+
+namespace App\Application\Actions\User;
+
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+
+class CreateUserAction extends UserAction
+{
+    /**
+     * {@inheritdoc}
+     */
+    protected function action(Request $request): Response
+    {
+        $parsedRequestBody = $request->getParsedBody();
+
+        $username =  $parsedRequestBody->username;
+        $firstname = $parsedRequestBody->firstname;
+        $lastname = $parsedRequestBody->lastname;
+
+        $user = $this->userRepository->createUser($username, $firstname, $lastname);
+
+        $this->logger->info("User has been created.");
+
+        return $this->respondWithData($user);
+    }
+
+}