relax UTF8 validation on generated clients

[tianshu-orcl]

when source data contains invalid UTF8 character data, the client code will raise error due to Unicode decoding error such as

    UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa0 in position 50555: invalid start byte

relax UTF8 validation can be done on the client. Each programming language has different approach. For example in python WebSocket client lib,

    from websocket import WebSocketApp

    ws = WebSocketApp(url,
              on_message=on_message,
              on_error=on_error,
              on_close=on_close,
              skip_utf8_validation=True)