add scheduler.dask_cluster_url traitlet, create dask cluster based on it

jupyter_scheduler/scheduler.py

@@ -7,6 +7,7 @@
 import fsspec
 import psutil
 from dask.distributed import Client as DaskClient
+from distributed import LocalCluster
 from jupyter_core.paths import jupyter_data_dir
 from jupyter_server.transutils import _i18n
 from jupyter_server.utils import to_os_path
@@ -402,6 +403,12 @@ class Scheduler(BaseScheduler):
         ),
     )
 
+    dask_cluster_url = Unicode(
+        allow_none=True,
+        config=True,
+        help="URL of the Dask cluster to connect to.",
+    )
+
     db_url = Unicode(help=_i18n("Scheduler database url"))
 
     task_runner = Instance(allow_none=True, klass="jupyter_scheduler.task_runner.BaseTaskRunner")
@@ -425,7 +432,10 @@ def __init__(
 
     def _get_dask_client(self):
         """Creates and configures a Dask client."""
-        return DaskClient()
+        if self.dask_cluster_url:
+            return DaskClient(self.dask_cluster_url)
+        cluster = LocalCluster(processes=True)
+        return DaskClient(cluster)
 
     @property
     def db_session(self):
@@ -786,7 +796,7 @@ async def stop_extension(self):
         Cleanup code to run when the server is stopping.
         """
         if self.dask_client:
-            self.dask_client.close()
+            await self.dask_client.close()
 
 
 class ArchivingScheduler(Scheduler):