A foundation for supporting usecase like B3, B4 and B5, involving minor refactoring in the core (#1512, #1187). First version of conditional mapping works as a transformation, where source fields have to be read ahead. More complex usecases require field reading to be dynamic, as well as collection index to be handled as a part of expression evaluation.

Epic Brief: https://docs.google.com/document/d/1ZQmZD4M6llnoWzC4gEf5LxQIaCcMs2TwtQVa_hTpoLo/edit?usp=sharing