User can set existing `securityGroup` on IPv6 VPC

[nikimanoledaki]

Part of #4255
Description

Eksctl allows users to set an existing SecurityGroup in their VPC with vpc.SecurityGroup. Ensure that this is also possible for IPv6 VPCs.

More info in the technical doc

Scope

• Replicate vpc.SecurityGroup IPv4 feature for IPv6 VPCs

Acceptance Criteria
As an eksctl user who has already created a securityGroup,
When I set ipFamily to IPv6 and my existing securityGroups in my config file,
And I create this cluster with eksctl,
I can see that my cluster has been created using that Security Group.

[Himangini]

might be possible already, validate that this works!!!! :D