Support multiple isolated cable apps

[rsamoilov]

Adds support for mounting multiple isolated Cable applications with independent namespaces and protocols.

Key changes:

• Rage::Cable.application now accepts namespace and protocol options
• Each isolated app uses its own connection class, channels, and protocol
• Broadcasts are relayed to all registered protocols

Example usage:

Rage.routes.draw do
  # Default cable app
  mount Rage::Cable.application, at: "/cable"

  # Isolated app with custom namespace and protocol
  mount Rage::Cable.application(
    namespace: AdminCable,
    protocol: :raw_websocket_json
  ), at: "/admin_cable"
end