message: Fix the order that `SyncConfig`'s members are declared in so that they
  match the order of the constructor arguments.
  `validate_sync_history` is not intended to be provided by clients so it
  is omitted from the constructor. This means it must be the last member
  in the struct.
diff: @@ -115,10 +115,11 @@ struct SyncConfig {
  bool client_validate_ssl = true;
  util::Optional<std::string> ssl_trust_certificate_path;
  std::function<sync::Session::SSLVerifyCallback> ssl_verify_callback;
  - bool validate_sync_history = true;
  bool is_partial = false;
  util::Optional<std::string> custom_partial_sync_identifier;
  + bool validate_sync_history = true;
  +
  // The URL that will be used when connecting to the object server.
  // This will differ from `reference_realm_url` when partial sync is being used.
  std::string realm_url() const;
  