Refactor: scheduler: make adding utilization a resource variant method

Previously, add_unallocated_utilization() behaved differently depending on the
resource variant, and pcmk_sched_utilization.c even included variant.h, which
are good signs that it should be a resource variant method.

This replaces add_unallocated_utilization(),
native_add_unallocated_utilization(), and group_add_unallocated_utilization()
with a new resource_alloc_functions_t method add_utilization().

This highlights that we never implemented adding utilization for bundles.