[gRPC] Pass --experimental_allow_proto3_optional to protoc in build.rs (#23226)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/sglang-grpc/build.rs

@@ -4,6 +4,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     tonic_build::configure()
         .build_server(true)
         .build_client(false)
+        .protoc_arg("--experimental_allow_proto3_optional")
         .file_descriptor_set_path(
             std::path::PathBuf::from(std::env::var("OUT_DIR").unwrap())
                 .join("sglang_descriptor.bin"),