Serialization/Deserialization enable custom validator method

[munemolx]

in default function, you expect that always we will need to validate the data o instance of type t.
but we might have cases where we also need to have more than type instance type validation, for example, in my case I want to serialize all dataclasses.dataclass to a dict using dataclasses.asdict.
So recommend adding the following enhancements to the register_type method .

def register_type(
    t: type[T],
    marker: str,
    encoder: Callable[[T], EncodedT],
    decoder: Callable[[EncodedT], T],
    validator: Optional[Callable[[T], bool]] = None
):
    """Add support for serializing/deserializing native python type."""
    _encoders[t] = (marker, encoder, validator)
    _decoders[marker] = decoder

  def default(self, o):
        reducer = getattr(o, "__json__", None)
        if reducer is not None:
            return reducer()

        if isinstance(o, textual_types):
            return str(o)

        for t, (marker, encoder, validator) in _encoders.items():
            if validator and validator(o):
                return _as(marker, encoder(o))
            if isinstance(o, t):
                return _as(marker, encoder(o))