Store and combine incremental response data

[calvincestari]

All responses (initial and incremental) are parsed through the parsers and executor. All data received prior to each response will need to be combined with the new data received so a complete response is formed - RFC section link.

The initial response data and data received in each incremental response will need to be retained and combined so that each incremental response can insert the latest received incremental response data at the correct path and return an up-to-date response to the request callback.

[calvincestari]

This was done in PR apollographql/apollo-ios-dev#217.

[github-actions]

Do you have any feedback for the maintainers? Please tell us by taking a one-minute survey. Your responses will help us understand Apollo iOS usage and allow us to serve you better.