feat: allow existing subnet public gateway attachment

[toddgiguere]

Description

Allow existing subnets to be attached to public gateways.

This change alters the input variable existing_subnet_ids to existing_subnets, and the type has changed from simple list of IDs to an array of objects containing the existing subnet ID and a boolean to direct attachment of public gateway.

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Release notes content

Allow existing subnets to be attached to public gateways.

• input variable existing_subnet_ids renamed to existing_subnets
• type of input chaged from list(string) to list(object)
• existing subnet object contains ID of subnet and boolean for public gateway attachment

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[toddgiguere]

/run pipeline

[terraform-ibm-modules-ops]

🎉 This PR is included in version 7.15.0 🎉

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