support for union types in struct104

Fixed parameter named from lowercase to CameCase in statemachine.install.

use the correct field serializer for the Encoding field by looking up field type instead of hardcoded 'Byte'

__bytes__ on the Buffer object is sometimes returning a bytearray instead of bytes.

Traceback (most recent call last):
  File "C:\Users\ddeason\PycharmProjects\opcua_test\venv\lib\site-packages\asyncua\client\ua_client.py", line 88, in _process_received_data
    data = bytes(buf)
TypeError: __bytes__ returned non-bytes (type bytearray)

* added fixmes about types
* added type hinting

* added cast to Index
* added fixmes about wrong type hinting in uaprotocol_auto.py

This commit closes #752.