support new endpoint: ProductSelection

[barbara79]

1. code and add tests that cover the created code. Your code should be warning-free.
   • [ ] tests existing
2. stick to PSR-2 and and don't reformat existing code.
   • [ ] code style check successful

Still missing:

• [ ] in-store/product-selection-assignments
• [ ] products/{id}/product-selections
• [ ] products/key=key/product-selections
• [ ] StoreAddProductSelectionAction, StoreChangeProductSelectionAction, StoreRemoveProductSelectionAction, StoreSetProductSelectionsAction
• [ ] ProductSelectionCreatedMessage, ProductSelectionDeletedMessage, ProductSelectionProductAddedMessage, ProductSelectionProductRemovedMessage, StoreProductSelectionsChangedMessage
• [ ] to modify StoreCreateMessage

[barbara79]

working on it in a separated branch