Checklist

• run npm run test and npm run benchmark (tests failing due to tsd - Can't expect type on a type intersection tsdjs/tsd#59)
• tests and/or benchmarks are included
• documentation is changed or added (docs in progress see branch typescript-docs)
• commit message and code follows Code of conduct

While working on the new documentation I realized that when defining generic properties for routes can be extremely repetitive, and exhaustive when only defining one of the 4 available generics (Body, Querystring, Params, Headers). For example:

import fastify, { 
  RouteShorthandOptions, 
  RequestBodyDefault, 
  RequestParamsDefault, 
  RequestHeadersDefault } from 'fastify'

const server = fastify()

type QuerystringType = {
  username: string,
  password: number
}

server.get<
  RequestBodyDefault,
  QuerystringType,
  RequestParamsDefault, 
  RequestHeadersDefault
>('/auth', async (request, reply) => {
  const { username, password } = request.query
  return `logged in!`
}) 

In this example we only need to modify the Querystring generic, but are required to pass in the preceding Body generic and the remaining Params, Headers, and Context generics.

This PR merges the 4 generics related to the request object into a single generic property RequestGeneric, allowing the developer to specify only the properties necessary using named properties. Thus, simplifying the previous example into:

import fastify from 'fastify'

const server = fastify()

type QuerystringType = {
  username: string,
  password: number
}

server.get<{ Querystring: QuerystringType }>('/auth', async (request, reply) => {
  const { username, password } = request.query
  return `logged in!`
}) 

Something going on in the tsd library we're using to test the type definitions is causing a previously passing test to fail so I have an issue opened to resolve that. We can consider this PR blocked until that is fixed.