EPIC: Handle independent subpackages in PackageRevisions

Independent Subpackage support is being implemented in kpt, see https://github.com/kptdev/kpt/issues/4347

This epic brings in kpt Independent Subpackage support into Porch and covers implementing Independent Subpackage handling in Porch PackageRevisions.

This epic is dependent on the current Porch extensions to kpt being available in kpt, see https://github.com/nephio-project/porch/issues/762