feat: KubeObject can choose fieldpath with "K=V" selector

[yuwenma]

Current GetXXX(fields []string) and SetXXX(fields []string) requires theinternal.variantof each fields to be a MapVariant except the last field fields[len(fields)-1]. This PR allows the intermediate variant to be a sliceVariant and select the element from the sliceVariant by a KEY=VALUE matching
For example, to get the target namespace from a ClusterRoleBinding object, the users can easily call
obj.Get(ns, "subjects", "kind=ServiceAccount", "namespace")
obj.Set(newNs, "subjects", "kind=ServiceAccount", "namespace")
ns := obj.GetStringOrDie("subjects", "kind=ServiceAccount", "namespace") // ns = example
obj.SetStringOrDie(newNs, "subjects", "kind=ServiceAccount", "namespace")

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: read-secrets-global
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: secret-reader
subjects:
- apiGroup: rbac.authorization.k8s.io
  kind: Group 
  name: admin 
  namespace: irrelevant # not chosen
- apiGroup: rbac.authorization.k8s.io
  kind: ServiceAccount
  name: admin
  namespace: example # target namespace