feat: Add custom decorator support

[R-Campbell]

This adds the ability to customize the Fastify decorator while defaulting to the existing swagger. This enables generating multiple swagger docs with different transform functions, allowing more flexibility for e.g. creating multiple docs based on custom route config options. I believe this solves #631 if the set of user scopes is known ahead of time.

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

[R-Campbell]

I'd love to get this in to be able to use it, anything that would help push it through?

[mcollina]

lgtm