**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
tests/cli/test_cli_parser.py
"""
        Test if each of cli.*_COMMANDS.arg name without conflict
        """
        subcommand = {
            var: cli_parser.__dict__.get(var)
            for var in cli_parser.__dict__
            if var.isupper() and var.startswith("COMMANDS")
        }
        for group, command in subcommand.items():
            for com in command:
                conflict_arg = [arg for arg, count in Counter(com.args).items() if count > 1]
                assert [] == conflict_arg, (
                    f"Command group {group} function {com.name} have " f"conflict args name {conflict_arg}"
                )
    def test_subcommand_arg_flag_conflict(self):
        """
        Test if each of cli.*_COMMANDS.arg flags without conflict
        """
        subcommand = {
            key: val
            for key, val in cli_parser.__dict__.items()
            if key.isupper() and key.startswith("COMMANDS")
        }
        for group, command in subcommand.items():
            for com in command:
                position = [