Add GKE Sandbox MicroVM runtime support to gke_tester and testcluster.

- Add RuntimeTypeMicroVM with kata-clh handler, microvm RuntimeClass
  overhead, node selector, and toleration configurations.
- Automatically configure nested virtualization when creating MicroVM
  nodepools.
- Add GKE API environment (--env) and endpoint override (--endpoint)
  support for testing against common GKE environments (prod, staging,
  staging2, test). User flags take precedence over environment variables.
- Store endpoint in TestRange and Cluster protos to allow locating
  and deleting clusters across environments.
- Calculate memory requests with headroom to satisfy node allocatable
  limits on GKE nodepools.

PiperOrigin-RevId: 970078173