Add idempotent to register_scheduler_plugin client (#6328)

Updates client-side interface of register_scheduler_plugin to pass through idempotent flag.

Extending #5545

Finalizing removal of kwargs support
deprecated in #5699
and removed in #6144

distributed/client.py

@@ -4418,16 +4418,14 @@ async def _get_task_stream(
         else:
             return msgs
 
-    async def _register_scheduler_plugin(self, plugin, name, **kwargs):
-        if isinstance(plugin, type):
-            plugin = plugin(**kwargs)
-
+    async def _register_scheduler_plugin(self, plugin, name, idempotent=False):
         return await self.scheduler.register_scheduler_plugin(
             plugin=dumps(plugin, protocol=4),
             name=name,
+            idempotent=idempotent,
         )
 
-    def register_scheduler_plugin(self, plugin, name=None):
+    def register_scheduler_plugin(self, plugin, name=None, idempotent=False):
         """Register a scheduler plugin.
 
         See https://distributed.readthedocs.io/en/latest/plugins.html#scheduler-plugins
@@ -4439,6 +4437,8 @@ def register_scheduler_plugin(self, plugin, name=None):
         name : str
             Name for the plugin; if None, a name is taken from the
             plugin instance or automatically generated if not present.
+        idempotent : bool
+            Do not re-register if a plugin of the given name already exists.
         """
         if name is None:
             name = _get_plugin_name(plugin)
@@ -4447,6 +4447,7 @@ def register_scheduler_plugin(self, plugin, name=None):
             self._register_scheduler_plugin,
             plugin=plugin,
             name=name,
+            idempotent=idempotent,
         )
 
     def register_worker_callbacks(self, setup=None):