Feature gap: Implement targetSizePolicy.mode to InstanceGroupManager

[Cezarus27]

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compute: added `target_size_policy.0.mode` field to `google_compute_instance_group_manager` resource (beta)

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

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[ScottSuarez]

failing to compile

--- see failing check

[Cezarus27]

failing to compile

--- see failing check

Project which runs tests needs to be added to allow list to be able to set target_size_policy.0.mode on creation. Update of this field is not available yet.

[ScottSuarez]

Well the issue is the library doesn't have the method mentioned

Error: google/services/compute/resource_compute_instance_group_manager.go:1295:64: undefined: compute.InstanceGroupManagerTargetSizePolicy
Error: google/services/compute/resource_compute_instance_group_manager.go:1300:18: undefined: compute.InstanceGroupManagerTargetSizePolicy
Error: google/services/compute/resource_compute_instance_group_manager.go:1637:56: undefined: compute.InstanceGroupManagerTargetSizePolicy

which is likely due to the field being unreleased. This library we use is generated from the discovery docs.