Allow a "cleanup" callback that is executed when a session ends

It would be nice to have something like this, sort of the opposite of a serve_layout() function that gets executed with app.layout for every new session. This would allow, for example, server-side storing (e.g. using Redis) that would delete session data when the user disconnects.

def serve_layout():
  user_id = uuid4()
  redis_db[user_id] = <some data here>
  ...
  return layout

app.layout = serve_layout

@dash.on_end_session
def cleanup_layout():
  del redis_db[user_id]

The issue names no files or tests; start by tracing how app.layout creates per-session state and how Dash registers lifecycle callbacks. Investigate the session-disconnect mechanism and define the callback's registration, execution, and failure behavior, then add coverage showing that session data is cleaned up when a session ends.