[Cable] Support stream_for/broadcast_to

[rsamoilov]

This pull request introduces support for channel-local streams in the Rage::Cable::Channel class, allowing channels to broadcast and subscribe to streams scoped to specific objects or identifiers.

Example:

class NotificationsChannel < Rage::Cable::Channel
  def subscribed
    stream_for current_user
  end
end

NotificationsChannel.broadcast_to(current_user, { message: "You have a new notification!" })