Support Custom Dimension with scope:item for GA4 imports

[AltamashShaikh]

Recently Google added a new scope for CustomDimensionitem, this scope is not supported in Matomo yet as we have only 2 types of scope action and visit, we an map this scope as action but there is a bug in core https://github.com/matomo-org/matomo/issues/19351 where action level custom dimensions are not reported correctly, we should wait for core to fix this first and accordingly add this new scope for GA4 imports