Fix protoc build error

crates/cdk-payment-processor/build.rs

@@ -1,5 +1,7 @@
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     println!("cargo:rerun-if-changed=src/proto/payment_processor.proto");
-    tonic_build::compile_protos("src/proto/payment_processor.proto")?;
+    tonic_build::configure()
+        .protoc_arg("--experimental_allow_proto3_optional")
+        .compile_protos(&["src/proto/payment_processor.proto"], &["src/proto"])?;
     Ok(())
 }