Implement Introspection Features

[eslingerbryan]

A big part of GraphQL's strength is the introspection capabilities of any given schema/API. Among other things, this will power the developer playground UI that will ship with this library to make development of GraphQL APIs easier over the full lifecycle of a product.

This is likely an epic-ish level issue that should be broken into smaller, more discrete issues before beginning development work.

https://spec.graphql.org/June2018/#sec-Introspection