Add response status to GraphQLResult

[calvincestari]

This work is to enable users to query the status of a response; whether it is a partial or complete response. The naming of the property is still undecided but 'response` is not a favourite.

[calvincestari]

After speaking with Anthony this isn't something we consider essential for a 'final' release of defer. It becomes more useful once the proposed defer RFC changes in response format are implemented.