Add support for ephemeral containers

[GuessWhoSamFoo]

This PR contains an initial pass at supporting ephemeral containers in Octant. If the feature gate is enabled when visiting a pod, it will automatically add debug container using a debian image.

Pods with an ephemeral container will have their object status modified as warning (yellow).

If there is interest in this feature, further efforts can be invested into making the debug container configurable and proper UI.

Which issue(s) this PR fixes

• Fixes Create ephemeral containers for debugging #1103

Special notes for your reviewer:
Example of a distroless image where sh and bash will not work:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      shareProcessNamespace: true
      containers:
      - name: nginx
        image: sfoo/nginx-distroless:latest
        ports:
        - containerPort: 80
        - containerPort: 443

Testing if a feature gate is enabled using mocked clients is not straightforward and I haven't figured out a way to do this yet. The terminal viewer component is pending a refactor soon because the terminal viewer does not have knowledge of the manager's state which causes unnecessary synchronization challenges.

Signed-off-by: GuessWhoSamFoo foos@vmware.com

[wwitzel3]

This looks pretty good, part of this should be a PR that lands in upstream as its own change though.