**Description**

Allow custom executors subclassed from existing (CeleryExecutor, CeleryKubernetesExecutor, etc.) to use default CLI commands to start workers or flower monitoring.

**Use case / motivation**

Currently, users who decide to roll their own custom Celery-based executor cannot use default commands (i.e. `airflow celery worker`) even though it's built on top of existing CeleryExecutor. If they try to, they'll receive the following error: `airflow command error: argument GROUP_OR_COMMAND: celery subcommand works only with CeleryExecutor, your current executor: custom_package.CustomCeleryExecutor, see help above.`

One workaround for this is to create custom entrypoint script for worker/flower containers/processes that are still going to use the same Celery app as CeleryExecutor. This leads to unnecessary maintaining of this entrypoint script.

I'd suggest two ways of fixing that:

- Check if custom executor is subclassed from Celery executor (which might lead to errors, if custom executor is used to access other celery app, which might be a proper reason for rolling custom executor)
- Store `app` as attribute of Celery-based executors and match the one provided by custom executor with the default one

**Related Issues**

N/A
airflow/cli/cli_parser.py
cmd = command._asdict()
    cmd['args'] = (arg for arg in command.args if arg is not ARG_DAG_ID)
    return ActionCommand(**cmd)
dag_cli_commands: List[CLICommand] = [
    GroupCommand(
        name='dags',
        help='Manage DAGs',
        subcommands=[
            _remove_dag_id_opt(sp)
            for sp in DAGS_COMMANDS
            if sp.name in ['backfill', 'list-runs', 'pause', 'unpause']
        ],
    ),
    GroupCommand(
        name='tasks',
        help='Manage tasks',
        subcommands=[_remove_dag_id_opt(sp) for sp in TASKS_COMMANDS if sp.name in ['list', 'test', 'run']],
    ),
]
DAG_CLI_DICT: Dict[str, CLICommand] = {sp.name: sp for sp in dag_cli_commands}
class AirflowHelpFormatter(argparse.HelpFormatter):
    """
    Custom help formatter to display help message.
    It displays simple commands and groups of commands in separate sections.