Part of #4255

Description
A user should be able to create an IPv6 cluster using their own IPv6 VPC by specifying the VPC ID in their config file (example of how we currently do this).

In terms of implementation, some validation is needed for the subnets and CIDR. More information about this can be found in the User supplied VPC config section in the technical design doc.

Scope

• Use an existing VPC given its ID in a similar way to how we do that for IPv4
• As is the case with IPv4, validate that the subnet has the minimum required number of subnets, unless privateCluster is enabled
• If the user has set the vpc.ipv6Cidr optional field, validate that they also set vpc.ipv6CidrPool
• Validate that the VPC has an Ipv6CidrBlockAssociationSet, plus that the IPv6 CIDR matches vpc.ipv6Cidr if that was set.

Acceptance Criteria
Story 1
As an eksctl user who already has an IPv6 VPC,
I can set my IPv6 VPC's ID and its subnets in my configuration file,
And when I create a cluster with eksctl,
I can see that my cluster has been created using that VPC, subnets, IPv6 CIDR, and IPv4 CIDR.

Story 2
As an eksctl user who already has an IPv6 VPC,
I can set my IPv6 VPC's ID, its subnets, and its IPv6 CIDR, in my configuration file,
And when I create a cluster with eksctl,
I can see that my cluster has been created using that VPC, subnets, IPv6 CIDR, and IPv4 CIDR.