**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_RUN_BACKWARDS = Arg(
    (
        "-B",
        "--run-backwards",
    ),
    help=(
        "if set, the backfill will run tasks from the most "
        "recent day first.  if there are tasks that depend_on_past "
        "this option will throw an exception"
    ),
    action="store_true",
)
ARG_SHOW_DAGRUN = Arg(
    ("--show-dagrun",),
    help=(
        "After completing the backfill, shows the diagram for current DAG Run.\n"
        "\n"
        "The diagram is in DOT language\n"
    ),
    action='store_true',
)
ARG_IMGCAT_DAGRUN = Arg(
    ("--imgcat-dagrun",),
    help=(
        "After completing the dag run, prints a diagram on the screen for the "
        "current DAG Run using the imgcat tool.\n"
    ),
    action='store_true',
)
ARG_SAVE_DAGRUN = Arg(