Add custom decorator support and async decorator support

[segevfiner]

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

The custom decorator support matches what is already in @fastify/swagger-ui, the async bit is for custom swagger decorator that might be async, which is not the case currently for those generated by @fastify/swagger for now.

[segevfiner]

WIll also need to handle the swaggerCSP decorator if we want to be able to use multiple instances without having to encapsulate them from each other.