feat: Control database subnet route table pattern

[gabrielhm1]

Description

Support for multiple database subnet architectures.

Motivation and Context

Before this change, when create_database_internet_gateway_route was set to true, the database subnet had the following behavior:

• For IPv4 traffic: 0.0.0.0 -> Internet Gateway
• For IPv6 traffic: ::/0 -> Egress-Only Internet Gateway

However, the Egress-Only Internet Gateway only allows outbound traffic. Consequently, the database subnet might receive inbound IPv4 traffic and outbound IPv4 traffic, but only outbound traffic for IPv6.

After this change, there are four patterns for the database subnet:

1. Public Pattern:

   • For IPv4 traffic: 0.0.0.0 -> Internet Gateway
   • For IPv6 traffic: ::/0 -> Internet Gateway

   create_database_subnet_route_table = true
   create_database_internet_gateway_route = true
   create_database_egress_only_internet_gateway_route = false
   create_database_nat_gateway_route = false

2. Full NAT Pattern:

   • For IPv4 traffic: 0.0.0.0 -> NAT Gateway
   • For IPv6 traffic: 64:ff9b::/96 -> NAT Gateway

   create_database_subnet_route_table = true
   create_database_internet_gateway_route = false
   create_database_egress_only_internet_gateway_route = false
   create_database_nat_gateway_route = true

3. IPv4 NAT + IPv6 Egress-Only:

   • For IPv4 traffic: 0.0.0.0 -> NAT Gateway
   • For IPv6 traffic: ::/0 -> Egress-Only Internet Gateway

   create_database_subnet_route_table = true
   create_database_internet_gateway_route = false
   create_database_egress_only_internet_gateway_route = true
   create_database_nat_gateway_route = true

4. IPv6 Egress-Only Pattern:

   • For IPv4 traffic: 0.0.0.0/0 -> Not configured
   • For IPv6 traffic: ::/0 -> Egress-Only Internet Gateway

   create_database_subnet_route_table = true
   create_database_internet_gateway_route = false
   create_database_egress_only_internet_gateway_route = true
   create_database_nat_gateway_route = false

Breaking Changes

If create_database_internet_gateway_route = true, "aws_route" "database_ipv6_egress" resource will be destroyed and replaced by "aws_route" "database_internet_gateway_ipv6".

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects
• I have executed pre-commit run -a on my pull request

The Terraform code was tested for all scenarios described in the motivation and context section.

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