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/

variables.tf

@@ -311,3 +311,12 @@ variable "eks_oidc_root_ca_thumbprint" {
   description = "Thumbprint of Root CA for EKS OIDC, Valid until 2037"
   default     = "9e99a48a9960b14926bb7f3b02e22da2b0ab7280"
 }
+
+variable "cluster_encryption_config" {
+  description = "Configuration block with encryption configuration for the cluster. See examples/secrets_encryption/main.tf for example format"
+  type = list(object({
+    provider_key_arn = string
+    resources        = list(string)
+  }))
+  default = []
+}