Add support for MQTT

[ccapndave]

Some GraphQL servers (specifically AWS AppSync) use MQTT for the websocket transport protocol, which unfortunally doesn't work with ApolloLinks.webSocketLink. It would be great if reason-apollo could support this link type too.

I believe this is the JS implementation of it: https://raw.githubusercontent.com/awslabs/aws-mobile-appsync-sdk-js/254930e3de1211c20bbac3efadf571008826bc32/packages/aws-appsync/src/link/subscription-handshake-link.js

I actually made an attempt to port this to Reason, but didn't quite manage to get it to work.

Issue Labels

• has-reproduction
• feature
• docs
• blocking
• good first issue

[ccapndave]

I found this (very hacky) workaround to use AWP AppSync. First add the aws-appsync package to the project. Then create the client using:

let client = [%bs.raw
  {|
(function() {
  let { AWSAppSyncClient, createAppSyncLink, AUTH_TYPE } = require("aws-appsync");

  return new AWSAppSyncClient(
    {
      url: "your graphql endpoint",
      region: "yourregion",
      auth: {
        type: AUTH_TYPE.API_KEY,
        apiKey: "yourapikey"
      }
    }
  );
})()
|}
];

After this you can use reason-apollo like usual, and subscriptions from AppSync work!