**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_USE_RANDOM_PASSWORD = Arg(
    ('--use-random-password',),
    help='Do not prompt for password. Use random string instead.'
    ' Required to create a user without --password ',
    default=False,
    action='store_true',
)
ARG_USER_IMPORT = Arg(
    ("import",),
    metavar="FILEPATH",
    help="Import users from JSON file. Example format::\n"
    + textwrap.indent(
        textwrap.dedent(
            '''
            [
                {
                    "email": "foo@bar.org",
                    "firstname": "Jon",
                    "lastname": "Doe",
                    "roles": ["Public"],
                    "username": "jondoe"
                }
            ]'''
        ),
        " " * 4,
    ),
)
ARG_USER_EXPORT = Arg(("export",), metavar="FILEPATH", help="Export all users to JSON file")
# roles
ARG_CREATE_ROLE = Arg(('-c', '--create'), help='Create a new role', action='store_true')