fix node serialization

[chkeita]

Summary of the Pull Request

The tuple type of Node.tasks is causing the following error when the field is deserialized

WARNING:root:unable to normalize type f<class 'tuple'>
WARNING:root:unable to normalize type f<class 'tuple'>
WARNING:root:unable to normalize type f<class 'tuple'>
Traceback (most recent call last):
  File "C:\temp\onefuzz\venv\Scripts\onefuzz-script.py", line 11, in <module>
    load_entry_point('onefuzz', 'console_scripts', 'onefuzz')()
  File "c:\work\onefuzz\src\cli\onefuzz\__main__.py", line 16, in main
    return execute_api(Onefuzz(), [Endpoint, Command], __version__)
  File "c:\work\onefuzz\src\cli\onefuzz\cli.py", line 591, in execute_api
    output(result, args.format, expression)
  File "c:\work\onefuzz\src\cli\onefuzz\cli.py", line 532, in output
    result = json.dumps(result, indent=4, sort_keys=True)
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\__init__.py", line 234, in dumps
    return cls(
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 201, in encode
    chunks = list(chunks)
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 325, in _iterencode_list
    yield from chunks
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 325, in _iterencode_list
    yield from chunks
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 438, in _iterencode
    o = _default(o)
  File "C:\Users\keita\AppData\Local\Programs\Python\Python38\lib\json\encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type NodeTaskState is not JSON serializable

This PR changes the type of that field to NodeTask which is supported by our serialization layer.