fix: add proto build flag to allow option proto fields

Signed-off-by: Kate Goldenring <[email protected]>

discovery-utils/build.rs

@@ -1,6 +1,7 @@
 fn main() {
     tonic_build::configure()
         .out_dir("./src/discovery")
+        .protoc_arg("--experimental_allow_proto3_optional")
         .compile(&["proto/discovery.proto"], &["proto"])
         .expect("failed to compile protos");
 }