Add EKS Secret envelope encryption support (#772)

This adds support for configuring EKS clusters that utilise envelope
encryption for Secrets:

- https://aws.amazon.com/about-aws/whats-new/2020/03/amazon-eks-adds-envelope-encryption-for-secrets-with-aws-kms/

- https://aws.amazon.com/blogs/containers/using-eks-encryption-provider-support-for-defense-in-depth/

versions.tf

@@ -2,7 +2,7 @@ terraform {
   required_version = ">= 0.12.9"
 
   required_providers {
-    aws        = ">= 2.44.0"
+    aws        = ">= 2.52.0"
     local      = ">= 1.2"
     null       = ">= 2.1"
     template   = ">= 2.1"