[All] Clear serialization language flag about incubator-fury

[LiangliangSui]

Yes, we can also keep it and add an explanation in xlang_spec. This will create an extra byte in the Fury Header. I'll update the spec later

[LiangliangSui]

cc @chaokunyang

[chaokunyang]

If the deserialization knows the encoding language, it may be able to do better encoding. For example, if sender is golang, and receiver is javascript, golang can use utf16 as the encoding instead of native utf8, then javascript deserialization will be faster.

[chaokunyang]

I think we can keep this currently, and add the design choice to the spec