[8.x] Enable only-to-others functionality when using Ably broadcast driver

[leeovery]

TL;DR

This PR enables the only to others broadcast functionality when using the Ably broadcast driver and the AblyJs frontend client.

Issue

When using the Ably broadcast driver the only to others functionality provided by Laravel does not work when using the AblyJs client on the frontend.

This is because Laravel includes the socketId in the payload array but Ably requires it to be a top-level property called connectionKey.

(source: https://faqs.ably.com/is-it-possible-to-prevent-messages-published-being-echoed-back-to-the-publishing-client)

Fix

This PR refactors the Ably broadcaster to pass a fully-formed Ably Message object to the Ably publish method, where we can also include the socketId as the connectionKey.

Currently, we pass a string event and an array payload to the publish method, and the Ably Rest client builds the Message object for us, but there is no way to include the connectionKey as a top-level property. (source: https://github.com/ably/ably-php/blob/main/src/Channel.php#L111)

To ensure backwards compatibility, the socket property will remain in the payload array too. It's possible people are working around this missing feature by using that property on the frontend.

With this PR merged, to enable the only to others functionality with Ably, you then only need to set the broadcast event to include the socket ID as per the Laravel docs, and then set echoMessages=false on the frontend when using the AblyJs client. This will prevent echoing messages back to originator.

Thanks
Lee

[leeovery]

To add, you also need to configure the axios interceptor. Echo isnt currently fully compatible with Ably. But thats a frontend issue and easily added:

eg:

const broadcaster = new Ably.Realtime({
  ...options,
  echoMessages: false,
});

let interceptorId;
broadcaster.connection.on('connected', () => {
 interceptorId = $axios.interceptors.request.use((config) => {
    const socketId = broadcaster.connection.key;
    if (socketId) {
      config.headers['X-Socket-Id'] = socketId;
    }

    return config;
  });
});
broadcaster.connection.on('suspended', () => {
  if (interceptorId) {
    $axios.interceptors.request.eject(interceptorId);
  }
});

[taylorotwell]

Don't we currently document using Ably via the Pusher JS client and Pusher protocol support enabled on the Ably control panel?

[leeovery]

Yea we do. But by using that you miss out on some Ably features in seems. They suggest not doing this too in their docs (obviously lol).

Point 3 under the General header: https://faqs.ably.com/using-the-ably-pusher-protocol-adapter

This PR wont break that functionality though as its purely additive. Just enables the feature for those that choose to go the Ably way in order to get the extra features and security.