Simplified manual member mapping

[Liamth99]

Describe the solution you'd like:
At the moment the only way to manually map a member is using the ForMember<T1, T2>(TargetProp) attribute.

[GenerateMapper]
[Map<User, UserDto>]
public partial class UserMapper
{
    [ForMember<User, UserDto>(nameof(UserDto.DayOfBirth))]
    public static int GetDayOfBirth(User source) => source.DateOfBirth.UtcDateTime.Day;
}

This can be simplified for simple property assignments so that they dont need to call a method. A signiture like ForMember<T1, T2>(SourceProp, TargetProp) would probably work.

[GenerateMapper]
[Map<User, UserDto>]
[ForMember<User, UserDto>(nameof(source.DateOfBirth.UtcDateTime.Day), nameof(UserDto.DayOfBirth))]
public partial class UserMapper;

Only issue is nameof() will return the name of the final property not the whole expression.