Create EKS admins group

[keyvaann]

Right now when creating an EKS cluster the access is given to the users individually, I think it's a better practice to create a group and add users to that group instead.
I updated the documentation, I hope it's not too vague.

[baixiac]

The maximum number of user groups a single user can belong to is hard-limited to 10 per region, which won't scale as the number of clusters grows. This limit hit us in the past so the idea of using the admin group was ditched in this commit. The other changes on README look good to me.

[keyvaann]

We can probably make this a configurable flag to either attach it to a group or a user.
For the hard limit issue, I suggest using vCluster if possible to reduce the number of the EKS clusters.
https://www.vcluster.com/

[baixiac]

Agreed on that flag can be provided to reflect users' preference for using either group or user policies. The virtual cluster solution is interesting. It looks like the virtualisation is done via creating underlying svc and port-forwarding when communicating with the EKS control plane. Their free pricing plan has limits on things like no. of (v)clusters and users, which could pose another bottleneck in terms of scalability?