fix: Revert removal of templates provider (#883)

Broke use case of passing in custom template content. Reverts most of the following PRs:
- #865
- #863 
- #854

examples/managed_node_groups/main.tf

@@ -19,6 +19,10 @@ provider "null" {
   version = "~> 2.1"
 }
 
+provider "template" {
+  version = "~> 2.1"
+}
+
 data "aws_eks_cluster" "cluster" {
   name = module.eks.cluster_id
 }