Create Plugins to invalidate StockItem indexes when Stock-to-Source assignments have been changed.

For now, we don't mark index as invalid after some changes happened in Sources-to-Stocks assignment.

Precondition:
Source A: assigned to Stock 1
Source B: assigned to Stock 2

after that, we decided to assign:
Source B: assigned to Stock 1

after this operation, we need to invalidate whole Stock 1 index, because all the StockItems for Stock 1 should be re-calculated.

It's a good architectural task because we need to invalidate not the whole indexes , but specific dimensions (Stocks) which were affected