feat: Allow lambda functions in mappings

[hwdegroot]

Add feature to use lambdas in fields_mappings, when registering a mapping. Gives the flexibility to define a mapping using the source object in a typesafe manner. similar to the c# AutoMapper.

class Property(BaseModel):
  x: str

class Source(BaseModel):
  default_index: int
  props: List[Property]

Target(BaseModel):
  default_prop: Property

mapper.map(Source, Target, fields_mapping={
  "default_prop": lambda source_obj: source_obj.props[source_ob.default_index]
})

In this mapping, The source class

source_obj = Source(default_prop=2, props=[Property(x="0"), Property(x="1"), Property(x="2"), Property(x="3")])
target_obj = mapper.map(source_obj)

will map to

Target(default_prop=Property(x="2"))

Normally one would do

source_obj = Source(default_prop=2, props=[Property(x="0"), Property(x="1"), Property(x="2"), Property(x="3")])
target_obj = mapper.to(Target).map(source_obj, fields_mapping={
  "default_prop": source_obj.props[source_ob.default_index]
})

But this is only possible as the moment the source_obj is initialized,.

[hwdegroot]

Hi @anikolaienko, really appreciate this library. Can you see if this can be merged into main Would provide a bit more functionality similar to the csharp AutoMapper library (and it would help me out).

Keep up the awsome work!