### Overview
Some Executors, currently a subset of the local Executors, run in a single threaded fashion and have certain limitations and requirements, many of which are hardcoded. To add a new single threaded Executor would require changes to core Airflow code.
Note: This coupling often shows up with SQLite compatibility checks since it does not support multiple connections.
### Examples
- **2a**) SQLite check done in configuration.py: https://github.com/apache/airflow/blob/26f94c5370587f73ebd935cecf208c6a36bdf9b6/airflow/configuration.py#L412-L419
- **2b**) When running in standalone mode SQLite compatibility is checked: https://github.com/apache/airflow/blob/26f94c5370587f73ebd935cecf208c6a36bdf9b6/airflow/cli/commands/standalone_command.py#L160-L165
- **2c**) Sensors in `poke` mode can block execution of DAGs when running with single process Executors, currently hardcoded to DebugExecutor (although should also include SequentialExecutor): https://github.com/apache/airflow/blob/27e2101f6ee5567b2843cbccf1dca0b0e7c96186/airflow/sensors/base.py#L243
### Proposal
A static method or attribute on the Executor class which can be checked by core code.

There is a precedent already set with the `supports_ad_hoc_ti_run` attribute, see:
https://github.com/apache/airflow/blob/fb741fd87254e235f99d7d67e558dafad601f253/airflow/executors/kubernetes_executor.py#L435 https://github.com/apache/airflow/blob/26f94c5370587f73ebd935cecf208c6a36bdf9b6/airflow/www/views.py#L1735-L1737
def test_poke_mode_only_allows_poke_mode(self):
        try:
            sensor = DummyPokeOnlySensor(task_id="foo", mode="poke", poke_changes_mode=False)
        except ValueError:
            self.fail("__init__ failed with mode='poke'.")
        try:
            sensor.poke({})
        except ValueError:
            self.fail("poke failed without changing mode from 'poke'.")
        try:
            sensor.change_mode("poke")
        except ValueError:
            self.fail("class method failed without changing mode from 'poke'.")
    def test_poke_mode_only_bad_class_method(self):
        sensor = DummyPokeOnlySensor(task_id="foo", mode="poke", poke_changes_mode=False)
        with pytest.raises(ValueError):
            sensor.change_mode("reschedule")
    def test_poke_mode_only_bad_init(self):
        with pytest.raises(ValueError):
            DummyPokeOnlySensor(task_id="foo", mode="reschedule", poke_changes_mode=False)
    def test_poke_mode_only_bad_poke(self):
        sensor = DummyPokeOnlySensor(task_id="foo", mode="poke", poke_changes_mode=True)
        with pytest.raises(ValueError):
            sensor.poke({})