Add custom field mapping into mapping registration

[anikolaienko]

I want to register custom field mappings:

class SourceClass:
    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age

class TargetClass:
    def __init__(self, name: str, age: int, profession: str):
        self.name = name
        self.age = age
        self.profession = profession

mapper.add(SourceClass, TargetClass, {"profession": "Software Engineer"})

source_obj = SourceClass("Andrii", 30)
target_obj = mapper.map(source_obj)

print(f"Name: {target_obj.name}; Age: {target_obj.age}; Profession: {target_obj.profession}")
# Output:
# Name: Andrii; age: 30; Profession: Software Engineer