**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_EMAIL,
            ARG_FIRSTNAME,
            ARG_LASTNAME,
            ARG_PASSWORD,
            ARG_USE_RANDOM_PASSWORD,
        ),
        epilog=(
            'examples:\n'
            'To create an user with "Admin" role and username equals to "admin", run:\n'
            '\n'
            '    $ airflow users create \\\n'
            '          --username admin \\\n'
            '          --firstname FIRST_NAME \\\n'
            '          --lastname LAST_NAME \\\n'
            '          --role Admin \\\n'
            '          --email admin@example.org'
        ),
    ),
    ActionCommand(
        name='delete',
        help='Delete a user',
        func=lazy_load_command('airflow.cli.commands.user_command.users_delete'),
        args=(ARG_USERNAME_OPTIONAL, ARG_EMAIL_OPTIONAL),
    ),
    ActionCommand(
        name='add-role',
        help='Add role to a user',
        func=lazy_load_command('airflow.cli.commands.user_command.add_role'),
        args=(ARG_USERNAME_OPTIONAL, ARG_EMAIL_OPTIONAL, ARG_ROLE),
    ),