api server lookups

[JimBugwadia]

Related issue

Fixes #1105

What type of PR is this?

Proposed changes

Add apiCall to context. Here is an example:

apiVersion : kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: limits
spec:
  validationFailureAction: enforce
  rules:
  - name: limit-pods
    match:
      resources:
        kinds:
        - Pod
    context:
    - name: podCount
      apiCall:
        urlPath: "/api/v1/namespaces/{{ request.namespace }}/pods"
        jmesPath: "items.length(@)"
    validate:
      message: "A maximum of 1 pod is allowed per namespace"
      deny:
        conditions:
        - key: "{{ podCount }}"
          operator: Equals
          value: 1
          

Checklist

• [x ] I have read the contributing guidelines.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments