feat: stream delta chunk

Co-Authored-By: Jan Kessler <Ithanil@users.noreply.github.com>

backend/open_webui/utils/payload.py

@@ -69,6 +69,7 @@ def remove_open_webui_params(params: dict) -> dict:
     """
     open_webui_params = {
         "stream_response": bool,
+        "stream_delta_chunk_size": int,
         "function_calling": str,
         "system": str,
     }