[4.0] Add enhanced client awareness

[jerelmiller]

This task is to add a clientLibrary key to each extensions key in the request body. This will be an object that contains name and version keys which will be populated with hard-coded values of the Apollo Client library and current version. Here is an example of the work done to support this in Apollo iOS.

This behaviour should be enabled by default but configurable so that users can opt-out if desired.