restructure types

[matthyk]

This PR aims to discuss this idea. I have 2 motivations for the restructuring:

• Previously, the types used internally were not exported. I think it is much more convenient if all types are simply exported, otherwise you have to stupidly rebuild the types for a TypeScript project
• I had strange problems registering the fastifyOverview plugin in my TypeScript project. After this restructuring I had no more problems

I have followed the example of other Fastify plugins for the restructuring. See for example @fastify/postgres or @fastify/mongodb.