Allow to retrieve authentication data for cluster

[baurmatt]

We'd like to work with the cluster created by this provider by leveraging other k8s specific Terraform providers like kubernetes/helm/helmfile/...

Please implement a solution which allows something like this:

Data Sources:

data "aws_eks_cluster" "cluster" {
  name = module.my-cluster.cluster_id
}

data "aws_eks_cluster_auth" "cluster" {
  name = module.my-cluster.cluster_id
}

provider "kubernetes" {
  host                   = data.aws_eks_cluster.cluster.endpoint
  cluster_ca_certificate = base64decode(data.aws_eks_cluster.cluster.certificate_authority.0.data)
  token                  = data.aws_eks_cluster_auth.cluster.token
  load_config_file       = false
  version                = "~> 1.9"
}

Cluster Sources Attributes:

• certificate_authority
• endpoint