Add GCP PostgreSQL IAM authentication support

[raviharshicorp]

• Add enable_iam_database_authentication variable for enabling IAM auth
• Add iam_database_user variable for service account email
• Update database module to support Cloud SQL IAM authentication
• Configure database_flags with cloudsql.iam_authentication=on
• Create IAM database user with CLOUD_IAM_SERVICE_ACCOUNT type
• Update database outputs to return appropriate user based on auth type
• Modify connection parameters to include authtype=gcp_iam for IAM auth
• Add postgres-passwordless example demonstrating IAM authentication
• Include service account creation and IAM role assignment in example
• Add comprehensive documentation for passwordless authentication

Relates : https://github.com/hashicorp/terraform-enterprise/pull/3177

How Has This Been Tested

CI/CD: https://github.com/hashicorp/terraform-enterprise/actions/runs/18259243048/job/51984887571