Support configuring runtimeClassName for instances

[kurt-google]

Is your feature request related to a problem? Please describe.
If we want to run oracle containers in a sandboxed runtime like gvisor https://cloud.google.com/kubernetes-engine/docs/how-to/sandbox-pods we need additional configuration functionality.

Describe the solution you'd like
It seems like the preferable way to implement this would be to expose an equivalent of pod.spec.runtimeClassName on our instance and propagate it to the statefulset.spec.template.spec.runtimeClassName field.

Describe alternatives you've considered
Another possible option is to implement the manual affinity and taint rules. This is possible with #268

Additional context