Add support for exec credentials for kubernetes and helm providers

[Lucasjuv]

Describe the solution you'd like
I want to be able to use exec credentials with the kubernetes provider.

Describe alternatives you've considered
I have considered installing the cli in the job before the terragrunt action but that didn't work.

Additional context

I get:

Error: Kubernetes cluster unreachable: Get "https://<guid>.gr7.us-east-1.eks.amazonaws.com/version": getting credentials: exec: executable aws not found
│ 
│ It looks like you are trying to use a client-go credential plugin that is not installed.
│ 
│ To learn more about this feature, consult the documentation available at:
│       https://kubernetes.io/docs/reference/access-authn-authz/authentication/#client-go-credential-plugins
│ 
│   with helm_release.argocd,
│   on argo.tf line 1, in resource "helm_release" "argocd":
│    1: resource "helm_release" "argocd" {

When I try to use these credentials:

data "aws_eks_cluster" "main" {
  name = var.cluster_name
}

provider "kubernetes" {
  host                   = data.aws_eks_cluster.main.endpoint
  cluster_ca_certificate = base64decode(data.aws_eks_cluster.main.certificate_authority[0].data)

  exec {
    api_version = "client.authentication.k8s.io/v1beta1"
    command     = "aws"
    args        = ["eks", "get-token", 
        "--cluster-name", data.aws_eks_cluster.main.name, 
        "--role-arn", var.deployer_iam_role_arn]
  }
}

provider "helm" {
  kubernetes {
    host                   = data.aws_eks_cluster.main.endpoint
    cluster_ca_certificate = base64decode(data.aws_eks_cluster.main.certificate_authority[0].data)

    exec {
      api_version = "client.authentication.k8s.io/v1beta1"
      command     = "aws"
      args        = ["eks", "get-token", 
        "--cluster-name", data.aws_eks_cluster.main.name, 
        "--role-arn", var.deployer_iam_role_arn]
    }
  }
}