feat: Allow factory method when defining a mapping

[hwdegroot]

Add feature to indicate a function or cunstructor to use as model factory. Mutually exclusive with the fields_mapping.
The model_factory: Callable[[S], T] will take a source_obj andshould return a target_obj

This allows the following mapping definitions

class SourceEnum(Enum):
    VALUE1 = "value1"
    VALUE2 = "value2"
    VALUE3 = "value3"

class NameEnum(Enum):
    VALUE1 = 1
    VALUE2 = 2
    VALUE3 = 3

class ValueEnum(Enum):
    A = "value1"
    B = "value2"
    C = "value3"


mapper.add(SourceEnum, NameEnum, model_factory=lambda x: NameEnum[x.name])
mapper.map(SourceEnum.VALUE1) # NameEnum.VALUE1

mapper.add(ValueEnum, SourceEnum, model_factory=lambda x: SourceEnum(x.value))
mapper.map(ValueEnum.B) #  SourceEnum.VALUE2


class ValueObject:
    value: str

    def __init__(self, value: Union[float, int, Decimal]):
        self.value = str(value)

mapper.to(ValueObject).map(Decimal("42"), model_factory=ValueObject) # ValueObject(42)