**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_POOL = Arg(("--pool",), "Resource pool to use")
ARG_DELAY_ON_LIMIT = Arg(
    ("--delay-on-limit",),
    help=(
        "Amount of time in seconds to wait when the limit "
        "on maximum active dag runs (max_active_runs) has "
        "been reached before trying to execute a dag run "
        "again"
    ),
    type=float,
    default=1.0,
)
ARG_RESET_DAG_RUN = Arg(
    ("--reset-dagruns",),
    help=(
        "if set, the backfill will delete existing "
        "backfill-related DAG runs and start "
        "anew with fresh, running DAG runs"
    ),
    action="store_true",
)
ARG_RERUN_FAILED_TASKS = Arg(
    ("--rerun-failed-tasks",),
    help=(
        "if set, the backfill will auto-rerun "
        "all the failed tasks for the backfill date range "
        "instead of throwing exceptions"
    ),
    action="store_true",
)