[FEATURE_REQUEST] Expand run-as-non-root template to verify runAsGroup field is nonzero.

[ariyonaty]

Description of the problem/feature request
In addition to it being a best security practice for pods to have runAsUser set to a non-zero value, it is also recommended that the GID, determined by either the runtime default security context or the runAsGroup field, is set to a non-zero value.

Would like to propose either creating a new template/check or extend the existing run-as-non-root template to check against the runAsGroup field.

Description of the existing behavior vs. expected behavior
Below is a snippet of behavior when runAsUser set to 0. Expected behavior would be along similar lines.

$ ./kube-linter lint ~/Documents/kube-linter/pkg/command/lint/testdata/valid-pod.yaml
KubeLinter 0.6.8

/home/user/Documents/kube-linter/pkg/command/lint/testdata/valid-pod.yaml: (object: <no namespace>/homebrew-demo /v1, Kind=Pod) container "homebrew-test" is not set to runAsNonRoot (check: run-as-non-root, remediation: Set runAsUser to a non-zero number and runAsNonRoot to true in your pod or container securityContext. Refer to https://kubernetes.io/docs/tasks/configure-pod-container/security-context/ for details.)

Error: found 1 lint errors

Additional context
Not particularly familiar with Go, but would be glad to take a crack at this.
Just let me know if there's any particular preference to either extend, or create new template, (or any other helpful suggestions/pointers).