### 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
for key, command, queue, executor_config in task_tuples:
            del self.queued_tasks[key]
            self.execute_async(key=key, command=command, queue=queue, executor_config=executor_config)
            self.running.add(key)
    def change_state(self, key: TaskInstanceKey, state: str, info=None) -> None:
        """
        Changes state of the task.
        :param info: Executor information for the task instance
        :param key: Unique key for the task instance
        :param state: State to set for the task.
        """
        self.log.debug("Changing state: %s", key)
        try:
            self.running.remove(key)
        except KeyError:
            self.log.debug("Could not find key: %s", str(key))
        self.event_buffer[key] = state, info
    def fail(self, key: TaskInstanceKey, info=None) -> None:
        """
        Set fail state for the event.
        :param info: Executor information for the task instance
        :param key: Unique key for the task instance
        """
        self.change_state(key, State.FAILED, info)
    def success(self, key: TaskInstanceKey, info=None) -> None: