Investigate the root `extensions` field on responses and Apollo cache control

[tomhoule]

Just a reminder/placeholder for discussion on these two features. The extensions field is probably part of the standard (TODO: check), and would be easy to implement - maybe as an Option<serde_json::Value> at first. Apollo cache control may be a cool feature in the concrete platform clients.

https://github.com/apollographql/apollo-cache-control