Automatic merge from submit-queue (batch tested with PRs 43870, 30302, 42722, 43736)

Admission plugin to merge pod and namespace tolerations for restricting pod placement on nodes

```release-note
This admission plugin checks for tolerations on the pod being admitted and its namespace, and verifies if there is any conflict. If there is no conflict, then it merges the pod's namespace tolerations with the the pod's tolerations and it verifies them against its namespace' whitelist of tolerations and returns. If a namespace does not have its default or whitelist tolerations specified, then cluster level default and whitelist is used. An example of its versioned config:

apiVersion: apiserver.k8s.io/v1alpha1
kind: AdmissionConfiguration
plugins:
- name: "PodTolerationRestriction"
  configuration:
    apiVersion: podtolerationrestriction.admission.k8s.io/v1alpha1
    kind: Configuration
    default:
     - Key: key1
       Value: value1
     - Key: key2
       Value: value2
    whitelist:
    - Key: key1
      Value: value1
    - Key: key2
      Value: value2
```