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

proto/pack.proto

@@ -10,6 +10,7 @@ message GitProtocolFeatures {
   repeated string capabilities = 2;
   repeated string server_options = 3;
   repeated string agent = 4;
+  bool stateless = 5;
 }
 
 message ReferenceAdvertisement {
@@ -24,11 +25,13 @@ message AdvertiseRefsRequest {
   RepositoryHeader repository = 1;
   GitProtocolFeatures protocol = 2;
   string service = 3;
+  bool raw = 4;
 }
 
 message AdvertiseRefsResponse {
   repeated ReferenceAdvertisement references = 1;
   repeated string capabilities = 2;
+  bytes raw_data = 3;
 }
 
 message UploadPackRequest {