Add utils command to migrate to pod identity associations

[TiberiuGC]

Description

eksctl introduces a new utils command for migrating existing IAM Roles for service accounts to pod identity associations, i.e.

eksctl utils migrate-to-pod-identity --cluster my-cluster --approve

Behind the scenes, the command will apply the following steps:

• install the eks-pod-identity-agent addon if not already active on the cluster
• identify all IAM Roles that are associated with K8s service accounts
• update the IAM trust policy of all roles, with an additional trusted entity, pointing to the new EKS Service principal (and, optionally, remove exising OIDC provider trust relationship)
• create pod identity associations between all identified roles and the respective service accounts

Running the command without the --approve flag will only output a plan consisting of a set of tasks reflecting the steps above, e.g.

[ℹ]  (plan) would migrate 2 iamserviceaccount(s) to pod identity association(s) by executing the following tasks
[ℹ]  (plan) 
3 sequential tasks: { install eks-pod-identity-agent addon, 
    2 parallel sub-tasks: { 
        update trust policy for owned role "eksctl-my-cluster-addon-iamserv-Role1-beYhlhzpwQte",
        update trust policy for unowned role "Unowned-Role1",
    }, 
    2 parallel sub-tasks: { 
        create pod identity association for service account "default/sa1",
        create pod identity association for service account "default/sa2",
    } 
}
[ℹ]  all tasks were skipped
[!]  no changes were applied, run again with '--approve' to apply the changes

Additionally, to delete the existing OIDC provider trust relationship from all IAM Roles, run the command with --remove-oidc-provider-trust-relationship flag, e.g.

eksctl utils migrate-to-pod-identity --cluster my-cluster --approve --remove-oidc-provider-trust-relationship

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟