[FEATURE_REQUEST] EnvVar ValueFrom checks

[josegomezr]

Description of the problem/feature request

A check to detect containers referring to configmaps/secret keys that do not exist in the current release via valueFrom (v1.EnvVarSource)

Description of the existing behavior vs. expected behavior

In some of our workloads we have containers with environment variables defined as:

# [...] clipped for brevity
- name: AIRBRAKE_PROJECT_KEY
  valueFrom:
    secretKeyRef:
      name: secrets
      key: airbrake-project-key
# [...] clipped for brevity

But it has happened a couple of times that a typo in the key slips and we notice it at the very last moment when the helm release fails to complete.

Additional context

The minimum reproduceable manifest could be something like:

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: example-deployment
  namespace: example
spec:
  selector:
    matchLabels:
      app.kubernetes.io/name: example
  template:
    metadata:
      labels:
        app.kubernetes.io/name: example
    spec:
      containers:
      - name: example-container
        image: internal.container.registry/an-image:v1
        env:
          - name: MY_DANGLING_ENV
            valueFrom:
              secretKeyRef:
                name: existing-secret
                key: with-a-typo # <-- notice here
---
apiVersion: v1
kind: Secret
metadata:
  name: example-deployment
  namespace: example
type: Opaque
data:
  without-a-typo: "YmFzZTY0Li4uCg=="

And the output should indicate which deployment or the container (whatever is more comfy) is referring to an unknown secret/config-map.

Currently there's no output of this, I do have a draft in a fork I can contribute to. I'm missing some tests but it sort-of works (golang is not my native speak):

main...josegomezr:kube-linter:env_source_check