feat: decorate fastify instance with addittional properties

[DemonHa]

Proposal regarding to #5061

We are decorating the FastifyInstance to take into account decorators so it can work together with fastifyPlugin in order to have to end to end type safe apis.

This PR is a draft so any suggestion will be highly appreciated.

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

[Uzlopak]

It feels kind of wrong to change the FastifyInstance to a type. Doesnt this result in potential issues because you maybe cant extend from a type?

Also i think AppInstance or app is a term of expressjs. A AppInstance or App in fastify is the FastifyInstance.

[DemonHa]

@Uzlopak Actually you can extend a type the same way you do with an interface

[DemonHa]

@Uzlopak If you are talking about declaration merging then you are correct, but this is the thing we are trying to avoid

[mcollina]

@DemonHa what can we do to help you ship this?

[DemonHa]

@DemonHa what can we do to help you ship this?

Getting feedback as early as possible would be a great help.

[DemonHa]

Was thinking how would it work with a plugin like autoload or even when defining routes on a separate function.

We would probably need a generic to tell us which plugins have been registered. Wdyt @mcollina ?