feat(core): Implement GraphQL SSE Response Support

[kitten]

Summary

This implements support for text/event-stream responses, as per GraphQL SSE.

We've used GraphQL Yoga's "push" mechanism as a general template for what this must look like: https://github.com/dotansimha/graphql-yoga/blob/d2060c1/packages/graphql-yoga/src/plugins/resultProcessor/push.ts
And this generally adheres to the GraphQL SSE protocol: https://github.com/enisdenjo/graphql-sse/blob/master/PROTOCOL.md

The current implementation found it unnecessary to respect event: type fields, instead it will just loosely look at data: fields and attempt to parse them.

We generally regard this as an alternative to multipart/mixed responses, and the protocol is loosely supported alongside it, while basically being an alternative to text/event-stream.
Currently, it's unclear to us when which one would be preferred, and we believe that's up to implementors and APIs to decide.

We also know that subscription operations could now be implemented with either response formats, however, we haven't decided yet how the default fetchExchange could switch over to handle subscription operations. This could be an option on the Client, but, we think that overall we already have a lot of fetch-related options on the Client, some of which may have to go. Discussion needed ✌️

5.9kB -> 5.98 kB min+gzip

Set of changes

• Add selector for response handlers
• Implement parseEventStream handler alongside parseMultipartMixed