KEP: Pod Schedule Readiness (kubernetes#3522)

* First version of KEP: Pod Schedule Readiness

* address comments including:

- reword `schedulePaused` to `schedulingPaused`
- support set spec.nodeName without clearing schedulingPaused
- add `schedulingGates` into UNRESOLVED
- add a story to support needs of array instead of a single boolean
- update the version skew section
- change the status from `provisional` to `implementable`

* fixup: rewordings

* fixup: major update

* minor: polish wordings

* address PRR comments

* address PRR comments (cont.)

* some rewordings

* address comments

Author: Huang-Wei

keps/prod-readiness/sig-scheduling/3521.yaml

@@ -0,0 +1,3 @@
+kep-number: 3521
+alpha:
+  approver: "@wojtek-t"