**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
ARG_STDERR,
            ARG_LOG_FILE,
            ARG_CAPACITY,
        ),
    ),
    ActionCommand(
        name='version',
        help="Show the version",
        func=lazy_load_command('airflow.cli.commands.version_command.version'),
        args=(),
    ),
    ActionCommand(
        name='cheat-sheet',
        help="Display cheat sheet",
        func=lazy_load_command('airflow.cli.commands.cheat_sheet_command.cheat_sheet'),
        args=(ARG_VERBOSE,),
    ),
    GroupCommand(
        name='connections',
        help="Manage connections",
        subcommands=CONNECTIONS_COMMANDS,
    ),
    GroupCommand(
        name='providers',
        help="Display providers",
        subcommands=PROVIDERS_COMMANDS,
    ),
    GroupCommand(
        name='users',
        help="Manage users",