[packages/sitecore-jss-react] Combine fetched and datasource item props in BYOC and FEAAS

[art-alexeyenko]

• This PR follows the Contribution Guide
• Changelog updated

Description / Motivation

This is a change to how FEEAS and BYOC handle fetched data.
FEAASComponent:

• ComponentDataOverride will only be used in server context. No change to functionality, since FEAASComponent is not used directly by Sitecore
• Data from item datasource is now always used by FEAAS, without overwriting anything from fetched data (rendering params or fetch endpoint)

BYOCComponent:

• Data from item datasource is now always used by BYOC, without overwriting anything from fetched data

Testing Details

• Unit Test Adjusted
• Manual Test/Other (Please elaborate)

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)

[Inviz]

I think this is good. good call on adding tests that check that both fields and data works together. I like the simplification of logic.

[ambrauer]

minor cleanup noticed otherwise 👍