[DRAFT] Whispering backport for Action Cable

[lHydra]

What is the purpose of this pull request?

Add whispering backport for Action Cable

Is there anything you'd like reviewers to focus on?

@palkan Hi! Could you please take a look at the MR? Did I understand the essence of the issue correctly?

That is, we want to have the following code in our Rails application:

import consumer from "channels/consumer"

consumer.subscriptions.create("ChatChannel", {
  whisper(payload) {
    this.perform("whisper", payload);
  }
})

Which will trigger the execution of the predefined #whisper method from the channel?

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated documentation

Closes #198