Added NAT Gateway IDs to the Subnets stats map

[j4zzcat]

There are scenarios where downstream components require the NAT Gateway IDs, e.g., when wiring the routing for a network firewall. Exposing them makes the subnet_stats_map more complete and has little risk.

Changes

• Added a local variables az_public_nat_gateway_ids_map that hold a map of NAT gatway IDs by AZ. For example:

az_public_nat_gateway_ids_map = {
  "us-east-1a" = ["nat-1", "nat-2"]
  "us-east-1b" = ["nat-3"]
}

• This is then added to the existing subnet stat maps, named_private_subnets_stats_map and named_public_subnets_stats_map.

Note: based on @aknysh code.