**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
"value you set for 'AIRFLOW_HOME' config you set in 'airflow.cfg' "
    ),
    default='[AIRFLOW_HOME]/dags' if BUILD_DOCS else settings.DAGS_FOLDER,
)
ARG_START_DATE = Arg(("-s", "--start-date"), help="Override start_date YYYY-MM-DD", type=parsedate)
ARG_END_DATE = Arg(("-e", "--end-date"), help="Override end_date YYYY-MM-DD", type=parsedate)
ARG_OUTPUT_PATH = Arg(
    (
        "-o",
        "--output-path",
    ),
    help="The output for generated yaml files",
    type=str,
    default="[CWD]" if BUILD_DOCS else os.getcwd(),
)
ARG_DRY_RUN = Arg(
    ("-n", "--dry-run"),
    help="Perform a dry run for each task. Only renders Template Fields for each task, nothing else",
    action="store_true",
)
ARG_PID = Arg(("--pid",), help="PID file location", nargs='?')
ARG_DAEMON = Arg(
    ("-D", "--daemon"), help="Daemonize instead of running in the foreground", action="store_true"
)
ARG_STDERR = Arg(("--stderr",), help="Redirect stderr to this file")
ARG_STDOUT = Arg(("--stdout",), help="Redirect stdout to this file")
ARG_LOG_FILE = Arg(("-l", "--log-file"), help="Location of the log file")
ARG_YES = Arg(
    ("-y", "--yes"), help="Do not prompt to confirm reset. Use with care!", action="store_true", default=False
)