Subscriptions Without Websockets

In building a recent B2B webapp, we discovered that some corporate networks block websockets. Similarly, some proxies cause websockets to appear open but do not allow any data through. Appsync cannot be used in these situations because its subscriptions rely on websockets without a fallback.

Appsync needs to support an alternative to websockets for passing real-time data through subscriptions where websockets cannot be used.

See [this Stack Overflow question](https://stackoverflow.com/questions/55873812/aws-appsync-subscriptions-without-websockets/55874376#55874376) for details.

The issue names no repository files, tests, or entry points. Start by reviewing the AppSync subscription behavior and the linked Stack Overflow discussion, then determine where subscription transport is defined and how a non-WebSocket path could be specified. Done means subscriptions work through networks and proxies that block or disrupt WebSockets.

Written by the indexing model from the issue text.