[Python] Support Optional typehint in pyfury

[chaokunyang]

Feature Request

Currently pyfury take all fields in a class as nullable:

@dataclass
class ComplexObject1:
    f1: Any = None
    f2: str = None
    f3: List[str] = None
    f4: Dict[pyfury.Int8Type, pyfury.Int32Type] = None
    f5: pyfury.Int8Type = None
    f6: pyfury.Int16Type = None
    f7: pyfury.Int32Type = None
    f8: pyfury.Int64Type = None
    f9: pyfury.Float32Type = None
    f10: pyfury.Float64Type = None
    f11: pyfury.Int16ArrayType = None
    f12: List[pyfury.Int16Type] = None

we should only take Optional fields as nullable.

In this way, we can mapping types like pyfury.Int32Type into primitive type int in java or int32 in golang directly

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