Pod Mutable Scheduling Directives

In #3521 we introduced PodSchedulingReadiness. The main use case for PodSchedulingReadiness was to enable building external resource controllers (like extended schedulers, dynamic quota managers) to make a decision on when the pod should be eligible for scheduling by kube-scheduler. This enhancement proposes to make the pod scheduling directives node selector and node affinity mutable as long as the pod is gated and the update to node selector/affinity further limits the existing constraints. This allows external resource controllers to not only decide when the pod should be eligible for scheduling, but also where it should land. This is similar to what we did for suspended Job in #2926. <!-- This section is incredibly important for producing high-quality, user-focused documentation such as release notes or a development roadmap. It should be possible to collect this information before implementation begins, in order to avoid requiring implementors to split their attention between writing release notes and implementing the feature itself. KEP editors and SIG Docs should help to ensure that the tone and content of the Summary section is useful for a wide audience. A good summary is probably at least a paragraph in length. Both in this section and below, follow the guidelines of the [documentation style guide]. In particular, wrap lines to a reasonable length, to make it easier for reviewers to cite specific portions, and to minimize diff churn on updates. [documentation style guide]: https://github.com/kubernetes/community/blob/master/contributors/guide/style-guide.md -->