improve autoscaling_policy support in google_compute_node_group

[modular-magician]

Fixes #13928

• node_group is now mutable
• Size is now an output only field
• inital_size or autoscaling_policy must be configured on resource creation

Release Note Template for Downstream PRs (will be copied)

compute: `size` in `google_compute_node_group` is now an output only field. 

compute: `google_compute_node_group` made mutable

compute: `google_compute_node_group` made to require one of `initial_size` or `autoscaling_policy` fields configured upon resource creation

Derived from GoogleCloudPlatform/magic-modules#9044

[github-actions]

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