feat(core): Support GraphQLRequest.extensions and update subscriptionExchange

[kitten]

Summary

This updates the GraphQLRequest type to contain an extensions field. This field will currently remain unavailable to bindings/UI code, but can be used to interact with spec extensions in exchanges.

Specifically, this enables us to replace persistedFetchExchange with a persitedExchange in the future that only interacts with the rest of the exchange pipeline by modifying Operation.extensions without triggering a fetch request.

The subscriptionExchange has been updated accordingly to receive the entire FetchBody type, to bring it inline with the default fetchExchange and @urql/core/internal utilities.

This is a breaking change as the properties key and context are now missing in the first argument that forwardSubscription receives. To compensate, the exchange now passes the entire Operation as a second argument to forwardSubscription. Generally this shouldn't affect integrations with libraries like graphql-ws.

Set of changes

• Replace subscriptionExchange's forwardSubscription input with FetchBody and add Operation as a second argument
• Add GraphQLRequest.extensions and expose it on createRequest