Add mock response option via TunnelOptions struct for local development

[dpkrn]

Add mock response option via TunnelOptions struct for local development

Feature Request

Summary:
Add support for mock responses during local development directly through the TunnelOptions struct.

Details:

• Introduce a Mock boolean field to TunnelOptions.
• When Mock is set to true, requests to endpoints should serve mock data instead of forwarding to the backend.
• Mock data should be configured in a mock.yml file at the project root, allowing per-endpoint mock responses.
• Users can enable mock support through:

  tunnelOptions := tunnel.TunnelOptions{
      Inspector:    true,
      InspectorAdd: "4040",
      Mock:         true,
  }

Mock file structure should be

mocks:
  - path: /api/user
    response:
      status: 200
      body: { "id": 1, "name": "Mock User" }
  - path: /api/orders
    response:
      status: 200
      body: [ { "orderId": 101, "amount": 10.99 } ]

Acceptance Criteria:

• Users enable mock data handling via TunnelOptions.
• The application reads mock responses from mock.yml.
• Documentation is provided illustrating configuration and usage.

Implementation Suggestions:

• Implement a loader for mock.yml and mapping from route to response.
• Middleware should check the Mock field and serve from configuration as appropriate.
• Provide sample mock.yml and update README.

Benefit: Makes it easier for developers to work independently when APIs or downstream services are not ready or offline.