feat: support providing multiple dns ips

[woehrl01]

Fixes #4836

Description

Updates all linux code paths to use all provided dns server ip addresses. Which are provided via the Ec2NodeTemplate.

How was this change tested?

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[heshamelsherif97]

This is a critical feature that needs to be supported, as there are valid use cases for using CoreDNS as a secondary cluster DNS in setups where node-local-DNS is in use.

It seems that the PR might be inadvertently breaking the isIPv6() function:

karpenter-provider-aws/pkg/providers/amifamily/bootstrap/eksbootstrap.go

Line 107 in 62cb956

func (e EKS) isIPv6() bool {

.

We should update the implementation to ensure IPv6 configurations remain unaffected.

[woehrl01]

@heshamelsherif97 Which concerns do you have about breaking this function? Of course, it could break it you pass in an ipv4 address in an ipv6 cluster for DNS resolution.

[heshamelsherif97]

I'm specifically referring to this line:

karpenter-provider-aws/pkg/providers/amifamily/bootstrap/eksbootstrap.go

Line 111 in 62cb956

return net.ParseIP(e.KubeletConfig.ClusterDNS[0]).To4() == nil

It appears that the current check assumes the first element in the list determines whether Karpenter should pass the --ip-family ipv6 argument to the bootstrap script:
https://github.com/aws/karpenter-provider-aws/blob/62cb956e8bc5348a50bfb77b3307d49741fc88ec/pkg/providers/amifamily/bootstrap/eksbootstrap.go#L69C30-L69C46

However, with this new change allowing multiple IPs to be provided, this approach may not be entirely correct in my humble opinion.