Allow request handlers to individually disable cors

[peterfaria-jitb]

I'm submitting this as a Draft PR to gather feedback on my approach.
I'm trying to allow routes to individually enable or disable CORS.
My team finds this useful for creating an API where not all routes
are desirable to be called from a browser.

My proposal is to allow a configuration option, cors: boolean.

Example usage:

  fastify.get<{  }, { cors: boolean }>(
    '/v1/foo/bar',
    {
      config: {
        cors: false,
      },
    },

Checklist

• run npm run test and npm run benchmark
• 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