[perf] optimize initialize for DPG model

[msyyc]

When DPG model initialize, we will call _get_deserialize_callable_from_annotation to loop all possible types to confirm the deserialization type. And there is same logic when [serialize].(

autorest.python/packages/typespec-python/test/azure/generated/special-words/specialwords/_model_base.py

Line 429 in 708bcb1

def _serialize(o, format: typing.Optional[str] = None): # pylint: disable=too-many-return-statements

)
Actually codegen already knows the type info when generated SDK so the runtime logic could be done when generation so that we could save the time for initialize. For example: we could add parameter serialize_func and deserialize_func for rest_field:

rfc3339: datetime.datetime = rest_field(serialize_func=serialize_rfc3339, deserialize_func=deserialize_rfc3339)