Implement Apollo WebSocket protocol

[Tobe Osakwe]

Subscription support is now available (not yet on Pub, also not yet thoroughly tested at all).
I intend to add some functionality to package:graphql_server that implements Apollo's WebSocket protocol:
https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md

This will be built on top of package:stream_channel (or even just dart:async, so that users of any framework/infrastructure can use it.

[Tobe Osakwe]

Of course, package:angel_graphql will have some functionality to make this easy to use within Angel.
Something like:

var graphQL = GraphQL(...);
app.get('/api/graphql_ws', graphQLWS(graphQL));

[Tobe Osakwe]

So, I've completed my implementation of the Apollo spec, but there's one issue... The Apollo spec is egregiously inaccurate, and completely out-of-sync with the format their actual client expects.

Follow this issue for updates:
apollographql/subscriptions-transport-ws#551

[Tobe Osakwe]

That being said, graphQLWS will be present in this next release of package:graphql_server. I am holding off on including unit tests until the referenced issue is addressed, as there's no sense in testing an incorrect spec.

The graphql_server docs will include information on how to set up a simple server, so that you can roll-your-own WebSocket support, on a case-by-case basis.

I am hesitant to define yet another protocol, so graphQLWS will rely on Apollo, at least for the foreseeable future.

[Tobe Osakwe]

I looked through their server code, and now the Angel implementation is consistent with theirs, though the spec is still very misleading.

The minimum version of subscriptions-transport-ws you can use is 0.8.3.