**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
args=(ARG_OUTPUT, ARG_VERBOSE),
    ),
    ActionCommand(
        name='get',
        help='Get pool size',
        func=lazy_load_command('airflow.cli.commands.pool_command.pool_get'),
        args=(ARG_POOL_NAME, ARG_OUTPUT, ARG_VERBOSE),
    ),
    ActionCommand(
        name='set',
        help='Configure pool',
        func=lazy_load_command('airflow.cli.commands.pool_command.pool_set'),
        args=(ARG_POOL_NAME, ARG_POOL_SLOTS, ARG_POOL_DESCRIPTION, ARG_OUTPUT, ARG_VERBOSE),
    ),
    ActionCommand(
        name='delete',
        help='Delete pool',
        func=lazy_load_command('airflow.cli.commands.pool_command.pool_delete'),
        args=(ARG_POOL_NAME, ARG_OUTPUT, ARG_VERBOSE),
    ),
    ActionCommand(
        name='import',
        help='Import pools',
        func=lazy_load_command('airflow.cli.commands.pool_command.pool_import'),
        args=(ARG_POOL_IMPORT, ARG_VERBOSE),
    ),
    ActionCommand(
        name='export',
        help='Export all pools',
        func=lazy_load_command('airflow.cli.commands.pool_command.pool_export'),