Automatic merge from submit-queue
Add PriorityClass API object under new "scheduling" API group
**What this PR does / why we need it**: This PR is a part of a series of PRs to add pod priority to Kubernetes. This PR adds a new API group called "scheduling" with a new API object called "PriorityClass". PriorityClass maps the string value of priority to its integer value.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**: Given the size of this PR, I will add the admission controller for the PriorityClass in a separate PR.
**Release note**:
```release-note
Add PriorityClass API object under new "scheduling" API group
```
ref/ #47604
ref/ #48646
This commit is an initial pass at providing an admission API group.
The API group is required by the webhook admission controller being
developed as part of https://github.com/kubernetes/community/pull/132
and could be used more as that proposal comes to fruition.
