Allow specifying NAT Gateways, subnets and routes when creating cluster.

[mvladev]

How to categorize this issue?

/area cost
/kind enchancement
/priority normal
/platform aws

What would you like to be added:

Allow for usage of pre-created AWS infrastructure as it would reduce costs (e.g. sharing NAT Gateways between clusters) and speed up the creation of the cluster. This would also further allow for full control over the network routing and design.

All resources must be tagged with "kubernetes.io/cluster/{{cluster-id}}" = "1" where cluster-id is shoot--{{project-name}}--{{shoot-name}} and the subnet used for internal LB should also be tagged with "kubernetes.io/role/internal-elb" = "use".

Why is this needed:

• Reduced costs
• Faster cluster creation
• Network customization