Add new /horse route which returns a horse sound

Description

Add a new endpoint /horse to the microservice that returns "neigh" when called with a GET request.

Technical Requirements

• Add new route /horse that accepts GET requests
• Implement response: "neigh"
• Follow existing project structure and coding standards
• Add appropriate unit tests
• Update readme

Implementation Details

Create new handler

• Add HorseHandler struct in handlers package

Update routing

• Add new route to router setup
• Group with existing endpoints under /api prefix

Testing

• Add unit tests for the new handler
• Test both possible responses
• Test invalid HTTP methods

Documentation

• Add example usage in README