Method to get list of mapped properties

Nobody has claimed this yet.

Mapstruct could generate based on the mapping define for one method, another method that for a given path return the target path.

Here is an example
```
class Car {
int seatCount;
}

class CarDto {
int numberOfSeats;
}

@Mapper
class CarMapper {
@Mapping(target="numberOfSeats", source="seatCount")
CarDto toCarDto(Car source);

--> @MappingDescriptor(method="toCarDto")
--> String carDtoProperty(String carPropertyPath);
}
```

Potential use case: When you define a rest api having a different name than the database model, you can use mapstruct to modify your pojo, but for the sort property you still have to change it manually. With this feature it could rely on how your map your pojo to map the sort property.

Start with the MapStruct annotation processor and the example CarMapper.toCarDto mapping; the issue names no source files or tests to inspect. Define how a generated carDtoProperty method should translate source property paths to target paths, including the completion criteria for mapped and unmapped paths.

Written by the indexing model from the issue text.