Add mock server support

[zhiguang-chen]

Add the ability to create and run a mock server within the app. This allows users to simulate API responses without needing a real backend, which is useful for testing, development, and demo purposes.

[zhiguang-chen]

[
  {
    "method": "GET",
    "path": "/users",
    "status": 200,
    "headers": { "Content-Type": "application/json" },
    "body": { "id": "{{uuid}}", "name": "John" },
    "delay": 300
  }
]

+ middleware