Annotations and Labels without values are not detected

[chonton]

For the resource below, I expect kubeconfig to complain about annotation values being empty (or null)

apiVersion: v1
kind: ConfigMap
metadata:
  name: some-values
  labels:
    some.domain/some-key:
  annotations:
    some.domain/some-key:
data:
  application.yaml: |-
    endpoints:
      service: https://example.com

Unfortunately, the ObjectMeta schema does not encode all requirements (or the schema validation package cannot detect missing values). I suspect that kubeconform would need to separately validate these requirements:

• https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-label-names
• https://kubernetes.io/docs/concepts/overview/working-with-objects/annotations/#syntax-and-character-set
• https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#syntax-and-character-set
• Total annotations size must be < 256KB:
  https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apimachinery/pkg/api/validation/objectmeta.go#L44-L67

Similar to #275, #286, and #240