[DTOSS-12318] - feat(iac): add nat-gateway module

[josielsouzanordcloud]

Description

This PR introduces a new nat-gateway module to deploy an Azure NAT gateway with a dedicated public IP address. The NAT gateway provides explicit outbound connectivity for subnets without public IP addresses, addressing the retirement of default outbound access by Microsoft.

Type of change

• Bug fix
• New feature
• Breaking change
• Infrastructure change
• Pipeline / CI
• Documentation update
• Refactor

Jira link

DTOSS-12318

Review notes

• README.md: Added comprehensive documentation for the NAT gateway module, including usage examples, prerequisites, and configuration options.
• main.tf: Defined resources for the NAT gateway, public IP, and associations with subnets. Utilised a submodule for creating the public IP address.
• outputs.tf: Exported outputs for the NAT gateway's ID, name, and associated public IP address.
• tfdocs.md: Generated Terraform documentation detailing required and optional inputs, resources used, and module outputs.
• variables.tf: Defined input variables with validations to ensure correct configuration.

Breaking changes

None

Impact

• Infrastructure: Adds support for deploying NAT gateways, enabling outbound connectivity for subnets without public IPs.
• Security: Enhances security by providing controlled outbound access through NAT gateways.
• Cost: Increases infrastructure costs due to the addition of NAT gateway and associated resources.

Review checklist

• No hardcoded secrets, credentials, or tokens
• PR title follows correct format (with ticket prefix if ticket provided, without if not)
• terraform plan reviewed — no unintended resource destruction
• Mandatory tags on all new resources
• Resource locks on critical resources
• Variable changes propagated to all environment .tfvars files
• State file impact assessed

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.