feat: new argument to use a custom external generator

[Shywim]

Q	A
Branch?	main
Tickets	#52
License	MIT
Doc PR	api-platform/docs#1469

This allow the -g/--generator argument to specify a path, allowing to use custom generators.

The argument can point to any node module.

A quick and dirty command to test is available as yarn test-gen-custom.

Notes:

• the lib entrypoint is now an async function because we do a dynamic import to load the custom generator inside src/generators.js

TODO:

• [ ] changelog
• [x] doc