Add Kubernetes client auth exec config support

[relu]

The implementation is similar to other Kubernetes Terraform providers
such as the kubectl or helm providers and allows the Flux provider to
authenticate against EKS clusters (or others supporting this method) by
configuring the client authentication exec plugin.

Example configuration for EKS:

provider "flux" {
  ...
  kubernetes {
    ...

    exec {
      api_version = "client.authentication.k8s.io/v1beta1"
      env         = {
        AWS_PROFILE = "example-profile"
      }
      command     = "aws"
      args        = [
        "--region",
        "us-east-1",
        "eks",
        "get-token",
        "--cluster-name",
        "example-cluster",
      ]
    }
  }
}

Closes: #406

[darkowlzz]

LGTM!

Left a few minor suggestions that can be addressed or ignored.

[stefanprodan]

LGTM

Thanks @relu 🥇