Adopt configurable Action Cable server

[samuel-williams-shopify]

Motivation

This builds on the proposed Rails server configuration hook in rails/rails#57803. Instead of inserting Async::Cable::Middleware into the Rails middleware stack while ActionCable.server remains a default ActionCable::Server::Base, this configures Action Cable itself to use an async server implementation.

That keeps the mounted Rack endpoint, ActionCable.server.broadcast, remote connections, pub/sub, heartbeat state, and channel internals on the same server object.

Changes

• Add Async::Cable::Server, a subclass of ActionCable::Server::Base that handles Rack WebSocket requests with async-websocket.
• Override #executor to use Async::Cable::Executor.
• Update the Railtie to set app.config.action_cable.server = Async::Cable::Server before Action Cable copies Rails config.
• Keep Async::Cable::Middleware available for compatibility, but no longer use it in the Railtie path.
• Point development/test Rails dependency at samuel-williams-shopify/rails, branch action-cable-configurable-server, so CI can exercise the pending Rails hook.
• Update README, getting started guide, gemspec comments, and release notes.

Verification

• bundle exec sus
• bundle exec rubocop
• bundle exec bake covered:validate
• bundle exec bake decode:index:coverage lib

Depends on rails/rails#57803.