message: Add get_context_free_data
diff: @@ -91,5 +91,14 @@ extern "C" {
  */
  time expiration();
  + /**
  + * Retrieve the signed_transaction.context_free_data[index].
  + * @param index - the index of the context_free_data entry to retrieve
  + * @param buff - output buff of the context_feee_data entry
  + * @param size - amount of context_free_data[index] to retrieve
  + * @return signed_transaction.context_free_data[index].size() or -1 if index not valid
  + */
  + int get_context_free_data( uint32_t index, char* buff, size_t size );
  +
  ///@ } transactioncapi
  }
  