### 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
task.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE, ignore_ti_state=True, **kwargs)
    def test_ok(self, make_sensor):
        sensor, dr = make_sensor(True)
        self._run(sensor)
        tis = dr.get_task_instances()
        assert len(tis) == 2
        for ti in tis:
            if ti.task_id == SENSOR_OP:
                assert ti.state == State.SUCCESS
            if ti.task_id == DUMMY_OP:
                assert ti.state == State.NONE
    def test_fail(self, make_sensor):
        sensor, dr = make_sensor(False)
        with pytest.raises(AirflowSensorTimeout):
            self._run(sensor)
        tis = dr.get_task_instances()
        assert len(tis) == 2
        for ti in tis:
            if ti.task_id == SENSOR_OP:
                assert ti.state == State.FAILED
            if ti.task_id == DUMMY_OP:
                assert ti.state == State.NONE
    def test_soft_fail(self, make_sensor):