[API] make primary key field indexes available:
- add QgsVectorLayer::pendingPkAttributeList()
- add QgsVectorDataProvider::pkAttributeIndexes()
- skip primary key attribute when pasting features (fixes #6164)
- default to skip primary key attribute in when merging features/attributes
- reset primary key attributes to default value when splitting features (better fix for #6060)
- comment out unused QgsPasteTransformations