feat: Add enable_public_igw_route flag to optionally create public route tables to internet gateway

Description

Add enable_public_igw_route flag to optionally create public route tables to internet gateway

Motivation and Context

It can be desirable to have both an internet gateway and want to alter the default routes for public subnets through either using vpc endpoints (gwlb) or transit gateways. Offering this simple flag offers flexibility and enables someone to output the public route table ids and add any routes they want, rather than prescribing to specific use cases.

This cant be inside the VPC module because the endpoints are created outside of it and use the private subnets so it would create a circular dependency. By adding enable_public_igw_route flag. it allows someone to alter these routes if wanted.

Example (pseudo)

resource "aws_route" "public_vpce_route" {
  for_each                       = aws_vpc_endpoint.gwlb
  route_table_id              = local.public_rtbl_to_subnet_id_map[each.key]
  destination_cidr_block = "0.0.0.0/0"
  vpc_endpoint_id           = each.value.id
}

Breaking Changes

This is fully backwards compatible and does not introduce any breaking changes.

How Has This Been Tested?

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

terraform-aws-modules / terraform-aws-vpc