Add gin router & health check endpoint
Adds the gin router and a health check endpoint at GET /health.

You can verify it still works by making a request to http://localhost:3000/api/v1/moon and you should see data that looks like this:

{
  "moon": {
    "age": 17.55,
    "distance_km": 405440.6286692368,
    "illumination": 91.495,
    "phase": "Waning Gibbous",
    "zodiac": {
      "sidereal": "gemini",
      "tropical": "cancer"
    }
  }
}