AWS EKS Add-on support

[shaun-nx]

AWS allows for software from independent software vendors to be installed as an Add-on to an EKS cluster on creation.

The aim is to have NGINX Ingress Controller available as an Add-on for EKS.

List of currently available add-ons from other independent software vendors can be found here: https://docs.aws.amazon.com/eks/latest/userguide/eks-add-ons.html#workloads-add-ons-available-vendors

### SPIKE, Design & POC and tasks
- [ ] https://github.com/nginxinc/kubernetes-ingress/issues/6001

### Implementation tasks
- [ ] https://github.com/nginxinc/kubernetes-ingress/issues/5996
- [ ] https://github.com/nginxinc/kubernetes-ingress/issues/6876

[github-actions]

Hi @shaun-nx thanks for reporting!

Be sure to check out the docs and the Contributing Guidelines while you wait for a human to take a look at this 🙂

Cheers!

[shaun-nx]

Moving the overall milestone to v4.1.0 as there some final things to Iron out and documentation to update