Namespace Selector for Pod Affinity

By default, pod affinity/anti-affinity constraints are calculated against the pods in the same namespace. The Spec allows users to expand that via the PodAffinityTerm.Namespaces list. This KEP proposes two related features: 1. Adding NamespaceSelector to PodAffinityTerm so that users can specify the set of namespace using a label selector. 1. Adding a new quota scope named CrossNamespaceAffinity that allows operators to limit which namespaces are allowed to have pods that use affinity/anti-affinity across namespaces. <!-- 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 -->