**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.dag_command.dag_list_dag_runs'),
        args=(
            ARG_DAG_ID_OPT,
            ARG_NO_BACKFILL,
            ARG_STATE,
            ARG_OUTPUT,
            ARG_VERBOSE,
            ARG_START_DATE,
            ARG_END_DATE,
        ),
    ),
    ActionCommand(
        name='list-jobs',
        help="List the jobs",
        func=lazy_load_command('airflow.cli.commands.dag_command.dag_list_jobs'),
        args=(ARG_DAG_ID_OPT, ARG_STATE, ARG_LIMIT, ARG_OUTPUT, ARG_VERBOSE),
    ),
    ActionCommand(
        name='state',
        help="Get the status of a dag run",
        func=lazy_load_command('airflow.cli.commands.dag_command.dag_state'),
        args=(ARG_DAG_ID, ARG_EXECUTION_DATE, ARG_SUBDIR),
    ),
    ActionCommand(
        name='next-execution',
        help="Get the next execution datetimes of a DAG",
        description=(
            "Get the next execution datetimes of a DAG. It returns one execution unless the "
            "num-executions option is given"