Propagating the Plugin type

[yuriy-yarosh]

So, I'm impl some basic TS typings for fastify-static in here #66. I've made a PR but @mcollina asked me explicitly to make fastify a development dependency.

I need a propagated fastify.Plugin type for that, and some generic type parameters like HttpServer / HttpRequest / HttpResponse. I've created the respective issue in fastify too #970.

Plugin type has too many type parameters, so I need to know if it's fine to drop the excessive ones first, before making any PR's.

[mcollina]

I think this could be closed, as some conclusion has been found in fastify/fastify#970.