**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 the name of cli.args mix option (-s, --long) valid
        """
        optional_mix = [
            arg for arg in cli_args.values() if len(arg.flags) == 2 and arg.flags[0].startswith("-")
        ]
        for arg in optional_mix:
            assert LEGAL_SHORT_OPTION_PATTERN.match(arg.flags[0]) is not None, f"{arg.flags[0]} is not match"
            assert ILLEGAL_LONG_OPTION_PATTERN.match(arg.flags[1]) is None, f"{arg.flags[1]} is not match"
    def test_subcommand_conflict(self):
        """
        Test if each of cli.*_COMMANDS without conflict subcommand
        """
        subcommand = {
            var: cli_parser.__dict__.get(var)
            for var in cli_parser.__dict__
            if var.isupper() and var.startswith("COMMANDS")
        }
        for group_name, sub in subcommand.items():
            name = [command.name.lower() for command in sub]
            assert len(name) == len(set(name)), f"Command group {group_name} have conflict subcommand"
    def test_subcommand_arg_name_conflict(self):