fix!: aurora postgres compatibility and sql server variable defaults

[nidahn]

Fix several issues that caused the module to be unusable for Aurora PostgreSQL when SQL Server features are not needed.

Changes:

• Remove rds_custom guard from master_password local — random password generation now works for Aurora Postgres
• Remove rds_custom guard from aws_db_parameter_group.this count — parameter group creation now works for Aurora Postgres
• Switch aws_security_group_rule.cidr_ingress (count + cidr_blocks list) to aws_vpc_security_group_ingress_rule with for_each — each CIDR is now an independent rule, future CIDR additions no longer force replacement of existing rules
• Add default = null to SQL Server-only variables (ad_domain_ou, ad_domain_fqdn, ad_domain_dns_ips, ad_domain_auth_secret_arn, db_group_name, option_name) — these no longer prompt for values when using Aurora Postgres
• Add default = [] to option_group and parameter_group variables

BREAKING CHANGE: aws_security_group_rule.cidr_ingress replaced with aws_vpc_security_group_ingress_rule using for_each. Existing deployments must perform state migration when upgrading:

1. terragrunt state rm 'aws_security_group_rule.cidr_ingress[0]'
2. terragrunt import 'aws_vpc_security_group_ingress_rule.cidr_ingress[""]' for each CIDR

[omerurhan]

LGTM