Adding new optional attribute routes into variable subnets.
This is useful in environments where core network, transit
gateways, etc. are created out of operator control.

Notes to the implementation:
We are limited here by Terraform because for_each works only with
set or map. As we have multiple subnets for a subnet / route table
we cannot use it. For that reason the code creates list of maps where
each map represents a route for the specific route table (attribute
route_table_name). In such case we can use count to iterate over it.

We are limited here by Terraform because for_each works only with
a set or map. As we have multiple routes for a subnet / route table,
we cannot use it. For that reason, the code creates a list of maps
where each map represents a route for the specific route table
(attribute route_table_name). In such case, we can use count to
iterate over it.

…oute

for 0.0.0.0/0 CIDR and connect_to_public_natgw set to true