Add support for the "extensions" key on errors

[mcasper]

The latest (published) graphql spec standardized an extensions key in the response format of graphql errors for optionally sending additional information about the error. The value of the extensions key is required to be a map, but beyond that there is no additional specification.

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

This PR adds support for graphql error extensions to the Graphql.Http.GraphqlError.GraphqlError record in the form of a Maybe (Dict String Json.Decode.Value) field, allowing users to handle the values however they please.