Add support to dual-stack ingress with bring-your-own VPC

[kingonion]

How to categorize this issue?

/area networking
/kind enhancement
/platform aws

What would you like to be added:
dual-stack ingress can support bring-our-own vpcs too.

Why is this needed:
We're trying to enable dual-stack ingress for our shoots by following https://github.com/gardener/gardener-extension-provider-aws/blob/master/docs/usage/dual-stack-ingress.md. But aws-load-balancer-controller failed to reconcile with following events.

35s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
33s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
31s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
30s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
28s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
27s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
25s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
23s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....
21s         Warning   FailedDeployModel      service/nginx-ingress-ingress-nginx-controller   Failed deploy model due to ValidationError: You must specify subnets with an associated IPv6 CIDR block....

I had an ask from Slack and was told that currently dual-stack ingress feature only support gardener managed vpcs. But hundreds of our shoots were created with bring-our-own vpc. We hope dual-stack ingress can support bring-our-own vpcs too.

The vpcs are created by our operators and the subnets are created by Gardener. We can assign Amazon managed CIDR range to VPC.