**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
func=lazy_load_command('airflow.cli.commands.task_command.task_render'),
        args=(ARG_DAG_ID, ARG_TASK_ID, ARG_EXECUTION_DATE_OR_DAGRUN_ID, ARG_SUBDIR, ARG_VERBOSE),
    ),
    ActionCommand(
        name='run',
        help="Run a single task instance",
        func=lazy_load_command('airflow.cli.commands.task_command.task_run'),
        args=(
            ARG_DAG_ID,
            ARG_TASK_ID,
            ARG_EXECUTION_DATE_OR_DAGRUN_ID,
            ARG_SUBDIR,
            ARG_MARK_SUCCESS,
            ARG_FORCE,
            ARG_POOL,
            ARG_CFG_PATH,
            ARG_LOCAL,
            ARG_RAW,
            ARG_IGNORE_ALL_DEPENDENCIES,
            ARG_IGNORE_DEPENDENCIES,
            ARG_IGNORE_DEPENDS_ON_PAST,
            ARG_SHIP_DAG,
            ARG_PICKLE,
            ARG_JOB_ID,
            ARG_INTERACTIVE,
            ARG_ERROR_FILE,
            ARG_SHUT_DOWN_LOGGING,
        ),
    ),
    ActionCommand(