feat: add `disconnect()` to node pool

pomice/pool.py

@@ -721,4 +721,9 @@ class NodePool:
 
         await node.connect()
         cls._nodes[node._identifier] = node
-        return node
+        return node
+
+    async def disconnect(self) -> None:
+        """Disconnects all nodes from the node pool."""
+        for node in self._nodes.copy().values():
+            await node.disconnect()