feat(core): Allow fetch exchange to handle subscriptions

[kitten]

Summary

This is a proposal which allows the fetchExchange to handle subscriptions.
This has been submitted to validate the following two assumptions:

• OperationContext.fetchSubscriptions & ClientOptions.fetchSubscriptions may or may not be a good idea
• GraphQL API frameworks like graphql-yoga will be able to handle subscriptions via text/event-stream/multipart/mixed response streams

If these two assumptions are fine, we can go ahead and merge this, I reckon.

Set of changes

• Add OperationContext.fetchSubscriptions & ClientOptions.fetchSubscriptions
  • Allow fetchExchange to handle subscriptions when enabled
• Switch Accept header when a subscription is sent