feat(pack): add raw advertise refs and stateless protocol support

- Add raw flag to AdvertiseRefsRequest to enable raw pkt-line output
- Implement advertise_refs_raw function that calls git upload-pack/receive-pack with --advertise-refs
- Add stateless flag to GitProtocolFeatures for HTTP smart protocol support
- Modify upload_pack and receive_pack to accept stateless parameter
- Update command construction to include --stateless-rpc flag when enabled
- Add raw_data field to AdvertiseRefsResponse for raw output
- Update pack cache key computation to include raw service differentiation
- Initialize raw field to false in all request creation calls

tests/refs_test.rs

@@ -20,6 +20,7 @@ async fn test_list_refs_via_service() {
             repository: Some(hdr()),
             protocol: None,
             service: String::new(),
+            raw: false,
         }))
         .await
         .unwrap()