Allow arbitrary env vars to be added to the Kellnr pod

[nyurik]

The AWS config sometimes requires additional environment variables to be defined in the pod, e.g. AWS_ACCESS_KEY_ID, AWS_ACCOUNT_ID, etc. There is no point to explicitly pass them one-by-one via Helm configuration because AWS could add more at any point, plus dealing with them is too tedious, error prone, and simply unnecessary. Instead, it would make sense to have a generic list of values that gets injected into the pod's environment:

values.yaml

# Extra environment variables to pass to the pod
# Users can override this to set arbitrary env vars
# Example:
# extraEnv:
#   - name: MY_CUSTOM_VAR
#     value: "my-value"
#   - name: MY_SECRET_VAR
#     valueFrom:
#       secretKeyRef:
#         name: my-secret
#         key: my-key
extraEnv: []

deployment.yaml

spec:
  template:
    spec:
      containers:
          env:
          {{- with .Values.extraEnv }}
          {{- toYaml . | nindent 10 }}
          {{- end }}