Part of #4255

Description

Users should be able to import their own v6 IPs into AWS and have them used as part of their VPC creation.

This is a slightly different UX from the current IPv4 workflow in which users can set a vpc.cidr with an actual CIDR and don't need to import anything beforehand.

Note: setting CIDR for IPv6 is a slightly different experience than for IPv4. For IPv4 users can provide whatever numbers (within reason) and no other work is required on their part. For IPv6, users have to do the work to import their CIDR into their account before they use eksctl. So there is an extra step that they will have to be made aware of in Docs, as the behaviour of this config is not quite the same as the equivalent IPv4 flag.

In order to support this for IPv6 we would have to expose 2 fields which would map to EC2::IPv6CidrBlock and EC2::IPv6Pool.

vpc:
  cidr: 192.0.2.0/24 <- users should still be able to set a v4 custom cidr
  ipv6Cidr: 2001:db8::/32 <- for the custom v6 CIDR
  ipv6Pool: <- must be set alongside ipv6Cidr

The rest of the new VPC setup is the same as detailed in the above section, the requested CIDR(s) will be set in the generated Cloud Formation template.

Scope

• The current vpc.cidr workflow should work with IPv6 as well. Users should be able to set both.
• If we validate v4 CIDRS then we need to validate v4 CIDRs as well.

Acceptance Criteria
Story 1

• As an eksctl user who has imported their own IPv6 IPs,
• I can set my custom IPv6 CIDR in my config file
• And when I create my cluster
• I see that my cluster and VPC have been created using that CIDR.

Story 2

• As an eksctl user who has imported their own IPv4 and IPv6 IPs,
• When I set IPv4 CIDR and IPv6 CIDR in my config file,
• And when I create my cluster
• I see that my cluster and VPC have been created using both given IPv4 and IPv6 CIDRs.