eks: add ephemeral aws_eks_cluster_auth resource

[bschaatsbergen]

Fixes #40343

Since it’s common to configure the Kubernetes provider by injecting a temporary IAM-compatible token for authenticating to the EKS control-plane (currently done using data.aws_eks_cluster_auth.example.token).

Terraform (1.10) supports referencing ephemeral resource attributes directly in providers. Having an ephemeral variant available of aws_eks_cluster_auth would greatly improve the security posture of Terraform users working with Amazon EKS and the Kubernetes or Helm provider as the temporary obtained IAM token is no longer persisted to the state.

ephemeral "aws_eks_cluster_auth" "example" {
  name = data.aws_eks_cluster.example.id
}

provider "kubernetes" {
  host                   = data.aws_eks_cluster.example.endpoint
  cluster_ca_certificate = base64decode(data.aws_eks_cluster.example.certificate_authority[0].data)
  token                  = ephemeral.aws_eks_cluster_auth.example.token
}

provider "helm" {
  kubernetes {
    host                   = data.aws_eks_cluster.example.endpoint
    cluster_ca_certificate = base64decode(data.aws_eks_cluster.example.certificate_authority[0].data)
    token                  = ephemeral.aws_eks_cluster_auth.example.token
  }
}

Test output:

 $ make testacc TESTARGS='-run=TestAccEKSClusterAuthEphemeral_basic' PKG=eks
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.23.3 test ./internal/service/eks/... -v -count 1 -parallel 20  -run=TestAccEKSClusterAuthEphemeral_basic -timeout 360m
2024/12/20 18:13:48 Initializing Terraform AWS Provider...
=== RUN   TestAccEKSClusterAuthEphemeral_basic
=== PAUSE TestAccEKSClusterAuthEphemeral_basic
=== CONT  TestAccEKSClusterAuthEphemeral_basic
--- PASS: TestAccEKSClusterAuthEphemeral_basic (11.50s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/eks        17.044s

[github-actions]

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