Title: feat(gce): add regional MIG support for multi-AZ instance groups

[wasimat404]

Note: This PR was developed with AI assistance during code exploration and drafting. I reviewed the code, ran builds and tests locally, and take responsibility for the implementation. Currently being revised based on maintainer feedback ,,,,moved to draft pending design direction.

Fixes #18411

tl;dr

There's been a TODO in AutoscalingGroupModelBuilder since 2017 about switching to Regional MIGs instead of splitting one InstanceGroup into separate per-zone zonal MIGs. The two blockers (no Terraform support, no zone steering) are both resolved now, so this PR does the thing.

what's in here

• New RegionInstanceGroupManager task — handles GCE API calls + renders as google_compute_region_instance_group_manager in Terraform with distribution_policy_zones
• New RegionInstanceGroupManagerClient in the compute client layer + mock for tests
• Build() refactored — when GCERegionalMIG feature flag is on, creates one regional MIG spanning all zones instead of N zonal MIGs
• Feature flag defaults to off so nothing changes for existing clusters

not breaking anything

Whole thing is behind KOPS_FEATURE_FLAGS=GCERegionalMIG. Flag off = old behavior, zero migration needed.

still todo

• Unit test for the regional path with flag enabled
• Would love maintainer input on the backwards compat approach before moving out of draft

[linux-foundation-easycla]

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[wasimat404]

/check-cla

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PR needs rebase.

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