Expose Extensions in Schema Introspection

[jasonbahl]

For tools like GraphiQL to make use of Extensions, I think there needs to be some Extension introspection of some sort, so that the extensions that exist can be known by tools like GraphiQL.

I envision GraphiQL having some interface, like a Muiti-Select that lets you choose which options you want to include in your request.

But for that to happen, GraphiQL would have to have knowledge of registered extensions, so that it could build out a Select with the options.

This ties in with #5 where a standard for dictating what extensions to include or exclude can be handled in the request in some fashion. (of course, server side rules could be configured to have certain extensions always on or always off, regardless of the client's request)