Add __repr__ for Executors (#13753)

Before:

```python
>>> from airflow.executors.local_executor import LocalExecutor
>>> LocalExecutor()
<airflow.executors.local_executor.LocalExecutor object at 0x7f49b47f8d68>
```

After:

```python
>>> from airflow.executors.local_executor import LocalExecutor
>>> LocalExecutor()
LocalExecutor(parallelism=32)
```

airflow/executors/base_executor.py

@@ -64,6 +64,9 @@ def __init__(self, parallelism: int = PARALLELISM):
         self.running: Set[TaskInstanceKey] = set()
         self.event_buffer: Dict[TaskInstanceKey, EventBufferValueType] = {}
 
+    def __repr__(self):
+        return f"{self.__class__.__name__}(parallelism={self.parallelism})"
+
     def start(self):  # pragma: no cover
         """Executors may need to get things started."""