add `extensions` field to GraphQLResponse<T>

[ikenox]

Hi, thank you for this really great library.

As described below, GraphQL response root may contain extensions field.

https://spec.graphql.org/June2018/#sec-Response-Format

The response map may also contain an entry with key extensions.

But GraphQLResponse<T> seems to have only data and errors field. I think It's better that we can access to extensions field via GraphQLResponse<T> class.