Support updating `iam_policy_arns` of an existing cluster

[deliahu]

[vishalbollu]

Updating iam_policy_arns requires changes to eksctl config.

There is an option of bypassing eksctl config and updating the nodeinstance roles manually. However this option becomes difficult to maintain because the iam_policy_arns changes need to be applied to new node instances roles belonging to newly added nodegroups as well.

We will postpone the implementation of this for now.