DINT-1856: Allow custom context for multiple ProductListItems

[NadiyaS]

Description

https://jira.corp.adobe.com/browse/DINT-1856

Add support of custom data in ProductListItems for events which may have several items:

• openCart
• viewCart
• addToCart
• removeFromCart
• initiateCheckout
• placeOrder
• addToRequisitionList
• removeFromRequisitionList

AC:

• add supporting custom data for ProductListItems for all events with next conditions:
  -- if there is only one ProductListItems in CustomContext which doesn't have SKU - merge this data with every ProductListItems
  -- if there are more than 1 ProductListItems or it has SKU - merge this data with ProductListItems which has the same SKU. 
• as SKU is used as identifier for merging, custom SKU for ProductListItems is not supported 

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.