**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_CFG_PATH = Arg(("--cfg-path",), help="Path to config file to use instead of airflow.cfg")
ARG_MIGRATION_TIMEOUT = Arg(
    ("-t", "--migration-wait-timeout"),
    help="timeout to wait for db to migrate ",
    type=int,
    default=0,
)
ARG_PORT = Arg(
    ("-p", "--port"),
    default=conf.get('webserver', 'WEB_SERVER_PORT'),
    type=int,
    help="The port on which to run the server",
)
ARG_SSL_CERT = Arg(
    ("--ssl-cert",),
    default=conf.get('webserver', 'WEB_SERVER_SSL_CERT'),
    help="Path to the SSL certificate for the webserver",
)
ARG_SSL_KEY = Arg(
    ("--ssl-key",),
    default=conf.get('webserver', 'WEB_SERVER_SSL_KEY'),
    help="Path to the key to use with the SSL certificate",
)
ARG_WORKERS = Arg(
    ("-w", "--workers"),
    default=conf.get('webserver', 'WORKERS'),
    type=int,
    help="Number of workers to run the webserver on",
)
ARG_WORKERCLASS = Arg(