thrift: add to_optional to union_field_ref

Summary: Tried to use it for union similarly to optional fields in D79896911 and realized it doesn't exist. I think it would make sense? To just match `value_or` -- it either has value or doesn't, so returning optional would make sense, thoughts?

Reviewed By: iahs

Differential Revision: D79907872

fbshipit-source-id: fa6a8161ec5aa8cdeb4bbdb1a427e48371f9ac24

Author: Pawel Poskrobko

third-party/thrift/src/thrift/lib/cpp2/FieldRef.h

@@ -1912,6 +1912,12 @@ class union_field_ref {
     return *this;
   }
 
+  FOLLY_ERASE std::optional<std::remove_const_t<value_type>> to_optional()
+      const {
+    using type = std::optional<std::remove_const_t<value_type>>;
+    return has_value() ? type(get_value()) : type();
+  }
+
   FOLLY_ERASE bool has_value() const { return type_ == field_type_; }
 
   FOLLY_ERASE explicit operator bool() const { return has_value(); }