Add uniqueness validation for subnetwork_ranges_list items

[annuay-google]

What?

Validate uniqueness of subnetwork_name field across subnetwork_ranges_list items.

Why?

An earlier PR had introduced a change to support subnetwork_ranges as a list in the interest of better templated blueprints. However, the uniqueness of the map structure was not maintained in the list, and hence would break when multiple items with same subnetwork_name are passed.

Testing

Passed two items with same subnetwork_name in the list, verified validation was triggered.

Submission Checklist

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• Add or modify unit tests to cover code changes
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• Update all applicable documentation
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