Allow specifying resource_policies in a disk created using instance template

[pbpatel93]

compute_instance_template resource allows attaching a list of resource policies for automatic snapshot creations:
https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_instance_template#disk

dynamic disk block in https://github.com/terraform-google-modules/terraform-google-vm/blob/master/modules/instance_template/main.tf#L71-L82 does not allow passing in resource_policies. This should be allowed and kept optional.

[github-actions]

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