Add InstanceGroup APIServer role for all provider

/kind feature

**1. Describe IN DETAIL the feature/behavior/change you would like to see.**

Currently the InstanceGroup APIServer role mentioned here https://kops.sigs.k8s.io/operations/scaling/ is locked to just AWS https://github.com/kubernetes/kops/blob/master/pkg/apis/kops/validation/instancegroup.go#L241

The goal is to support APIServer only instances on all providers (e.g. GCP).

**2. Feel free to provide a design supporting your feature request.**

Start with pkg/apis/kops/validation/instancegroup.go at the referenced validation logic, then read operations/scaling/ for the documented InstanceGroup APIServer role. Trace how provider-specific validation is applied and compare the provider implementations involved. Done means APIServer-only InstanceGroups are accepted and supported consistently across providers, with relevant validation or provider tests updated.

Written by the indexing model from the issue text.