code: void writeExtensionByte(int extensionType, int value) throws IOException {
          output.write(FIX_EXT_1);
          output.write(extensionType);
          output.write(value);
      }
docs: Writes an extension string using FIX_EXT_1.
  
  @param extensionType  the type
  @param value  the value to write as the data
  @throws IOException if an error occurs