Feature request: Typed services, controllers, etc.

[Alexnortung]

Problem

When working with services, you might define your own signatures for functions, which currently is not used or type checked.

For example if I use:

const result = strapi.plugin(pluginId).serice('myService').findOne(1)

I would like findOne to type check that it's parameter should be an id.
I would also like that result would have the type that my function returns.

Possible solution

The idea is that you could provide the plugins/services/controllers as a generic to the IStrapi interface. It would maybe look something like this in practice

import myService from './services/myService'

type MyStrapi = IStrapi<{
  plugins: {
    myPlugin: {
      services: {
        myService: typeof myService
      }
    }
  }
}>

We could also provide more types to populate even more types

[boazpoolman]

+1 for this feature. Would be great to have typed services.