Add typing to Azure Cosmos Client Hook (#23941)

New release of Azure Cosmos library has added typing information
and it broke main builds with mypy verification.

airflow/providers/microsoft/azure/hooks/cosmos.py

@@ -86,7 +86,7 @@ def get_ui_field_behaviour() -> Dict[str, Any]:
     def __init__(self, azure_cosmos_conn_id: str = default_conn_name) -> None:
         super().__init__()
         self.conn_id = azure_cosmos_conn_id
-        self._conn = None
+        self._conn: Optional[CosmosClient] = None
 
         self.default_database_name = None
         self.default_collection_name = None