This abstraction moves the responsibility of merging topology hints to
the individual policies themselves. As part of this, it removes the
CanAdmitPodResult() API from the policy abstraction, and rolls it into a
second return value from Merge()
These policies only differ on whether they admit the pod or not when a
TopologyHint is preferred or not. As such, the restricted policy should
simply inherit whatever it can from the best effort policy and only
overwrite what is necessary.
This does not matter for now, but will become important when we add a
new 'Merge()' abstraction to a Policy later on.
Previously it only took a bool, which limited the logic it could perform
to determine if a pod should be admitted or not based on the merged hint
from the policy.
