feat: support route-level CORS configuration

[vimutti77]

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

This PR adds support for configuring CORS on a per-route basis via config.cors. This allows developers to override or disable CORS behavior for specific routes, while still using the global plugin configuration as a default.

const fastify = require('fastify')()

fastify.register(require('@fastify/cors'), { origin: 'https://example.com' })

fastify.get('/cors-enabled', (_req, reply) => {
  reply.send('CORS headers applied')
})

fastify.get('/cors-allow-all', {
  config: {
    cors: {
      origin: '*', // Allow all origins for this route
    },
  },
}, (_req, reply) => {
  reply.send('Custom CORS headers applied')
})

fastify.get('/cors-disabled', {
  config: {
    cors: false, // Disable CORS for this route
  },
}, (_req, reply) => {
  reply.send('No CORS headers')
})

fastify.listen({ port: 3000 })