(experimental) - Add full support for multipart/mixed defer and stream directive responses

[kitten]

Add experimental support for @defer and @stream responses for GraphQL. This implements the "GraphQL Defer and Stream Directives" and "Incremental Delivery over HTTP" specifications. If a GraphQL API supports multipart/mixed responses for deferred and streamed delivery of GraphQL results, @urql/core (and all its derived fetch implementations) will attempt to stream results. This is only supported on browsers supporting streamed fetch responses, which excludes IE11.

The implementation of streamed multipart responses is derived from meros by @maraisr, and is subject to change if the RFCs end up changing.

For more information, read: https://graphql.org/blog/2020-12-08-improving-latency-with-defer-and-stream-directives

On fresh builds with a quick gzip check across all minified files ($ cat packages/core/dist/*.min.mjs | gzip-size) we see an increase from 5.32kB minzipped to 6.02kB minzipped, meaning a delta of approximately 700B.
On Graphcache ($ cat exchanges/graphcache/dist/*.min.mjs | gzip-size) we see a change from 9.38kB to 9.55kB, a delta of 170B.
Therefore we can estimate that the total impact of this feature is limited to <1kB minzipped.

[changeset-bot]

🦋 Changeset detected

Latest commit: 91b6650

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@urql/exchange-graphcache	Minor
@urql/core	Minor
@urql/exchange-execute	Minor

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[kitten]

We've released initial prerelease versions:

Package	Version
@urql/core@defer	2.3.0-defer.0
@urql/exchange-graphcache@defer	4.3.0-defer.0
@urql/exchange-execute@defer	1.1.0-defer.0

This will automatically enable this PR's patch. It may be necessary to create a resolution in Yarn, e.g. "resolutions": { "@urql/core": "2.3.0-defer.0" }, for the core package to remain deduplicated and on the new version.