**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
name='kerberos',
        help="Start a kerberos ticket renewer",
        func=lazy_load_command('airflow.cli.commands.kerberos_command.kerberos'),
        args=(ARG_PRINCIPAL, ARG_KEYTAB, ARG_PID, ARG_DAEMON, ARG_STDOUT, ARG_STDERR, ARG_LOG_FILE),
    ),
    ActionCommand(
        name='webserver',
        help="Start a Airflow webserver instance",
        func=lazy_load_command('airflow.cli.commands.webserver_command.webserver'),
        args=(
            ARG_PORT,
            ARG_WORKERS,
            ARG_WORKERCLASS,
            ARG_WORKER_TIMEOUT,
            ARG_HOSTNAME,
            ARG_PID,
            ARG_DAEMON,
            ARG_STDOUT,
            ARG_STDERR,
            ARG_ACCESS_LOGFILE,
            ARG_ERROR_LOGFILE,
            ARG_ACCESS_LOGFORMAT,
            ARG_LOG_FILE,
            ARG_SSL_CERT,
            ARG_SSL_KEY,
            ARG_DEBUG,
        ),
    ),
    ActionCommand(
        name='scheduler',