use "proto3_optional" option as signal that a field has presence.

This is technically redundant with the synthetic oneof container, but simplifies some tooling.

PiperOrigin-RevId: 604368850

java/core/src/main/java/com/google/protobuf/Descriptors.java

@@ -1443,7 +1443,8 @@ public final class Descriptors {
       if (isRepeated()) {
         return false;
       }
-      return getType() == Type.MESSAGE
+      return isProto3Optional
+          || getType() == Type.MESSAGE
           || getType() == Type.GROUP
           || getContainingOneof() != null
           || this.features.getFieldPresence() != DescriptorProtos.FeatureSet.FieldPresence.IMPLICIT;