feat: Add openAPIEnabled property to route metas and route rules

[horvbalint]

🔗 Linked issue

#2889

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds a new property openAPIEnabled to the NitroRouteMeta and NitroRouteConfig types. Using this property one can include/exclude specific routes in the generated _openapi.json.

To only include a part of the backend one can do for example:

export default defineNitroConfig({
  experimental: {
    openAPI: true,
  },
  routeRules: {
    "*": {
      openAPIEnabled: false,
    },
    "/api/public/**": {
      openAPIEnabled: true,
    },
  },
});

It is also possible to override this rule using the routes meta, like:

// /api/public/exceptionRoute.ts

defineRouteMeta({
  openAPIEnabled: false
})

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.