update rover's introspection to the new version of apollo-rs libraries

[lrlna]

Rover currently uses apollo-encoder to serialise incoming serde_json introspection result into SDL. With the new introspection and execution code, and the upcoming 1.0.0 release of apollo-compiler we need to update Rover to a new way of encoding serde_json to SDL.

[SimonSapin]

As far as I can tell for this purpose rover has:

• A set of Rust types with derive(serde::Deserialize) that match the structure of an introspection JSON response. I think this can stay unchanged
• Conversion from the above types to apollo-encoder types, for serialization to SDL. I think this can be swapped with conversion to either Schema or ast::Document from apollo-compiler v1.0. They both also support serialization to SDL

[SimonSapin]

In PR #758 we recently introduced "server-side" support for introspection in apollo-compiler. So I think it’d make sense to provide this "client-side" functionality in apollo-compiler as well, not just in Rover.

[goto-bus-stop]

I gave this a quick shot a few weeks back (https://github.com/apollographql/rover/tree/apollo-1.0) until I hit #761 and figured it was maybe not our top priority. But it should not be a lot of work to complete.

Moving (part of) that into apollo-rs could definitely make sense

[lrlna]

If nothing extraordinary comes up, I think we can safely postpone this work til February next year.