feat: add possibility to use ACM certificates

[Dariocent]

Note

Introduces optional TLS termination via existing ACM certificates and updates generated Helm configs accordingly.

• Adds acm_certificate_arn input (with ARN validation); trims/propagates to locals and template rendering in main.tf
• templates/helm-values-bootstrap.tpl.yaml: when acm_certificate_arn is set, configures ingress-nginx Service for NLB with SSL annotations and HTTP backend
• templates/helm-values.tpl.yaml: conditionally removes cert-manager clusterIssuer and tls sections when ACM is used; otherwise preserves Let’s Encrypt defaults
• Docs: updates CHANGELOG.md (v0.13.0) and README Inputs table to document the new option

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[diegofiori]

@cursor review