Core Kubernetes class structure

[zkck]

This PR includes the core class hierarchy for the Kubernetes/GKE functionality that we'd like to add to CFU.

Overview of the class hierarchy:

K8sClient (abstract)  // All Kubernetes objects
├── K8sCluster
└── K8sResource (abstract)  // Named Kubernetes objects
    ├── K8sNamespacedResource (abstract)  // Named Kubernetes objects within a namespace
    │   ├── K8sPod
    │   └── K8sWorkload
    │       └── K8sDeployment
    └── K8sNode

Example usage:

from libcloudforensics.providers.gcp.internal.gke import GkeCluster
from libcloudforensics.providers.kubernetes.base import K8sCluster
from libcloudforensics.providers.kubernetes.workloads import K8sDeployment

gke_cluster = GkeCluster('project-id', 'zone-id', 'cluster-id')
api_client = gke_cluster.GetCredentials()
k8s_cluster = K8sCluster(api_client)

print()
print('Pods in namespace "default":')
for pod in k8s_cluster.ListPods('default'):
  print(pod.name, pod.namespace, pod.GetNode().name)

print()
print('Pods in namespace "kube-system":')
for pod in k8s_cluster.ListPods('kube-system'):
  print(pod.name, pod.namespace, pod.GetNode().name)

print()
print('Pods in all namespaces:')
for pod in k8s_cluster.ListPods():
  print(pod.name, pod.namespace, pod.GetNode().name)

print()
print('Pods in nginx deployment')
for pod in K8sDeployment(api_client, 'nginx', 'default').GetCoveredPods():
  print(pod.name, pod.namespace, pod.GetNode().name)

Still in draft phase since mock tests must be added.

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[zkck]

Quick note, I wasn't sure of the best placement for the kubernetes package. It's under providers right now, I'm open to suggestions.