Add some default tags to the AWS resources for the EKS cluster.

This adds some of the tags from the [tagging guide] from the previous
ECS project. I've omitted `chargeable_entity` and `environment` because
those might need a bit of a rethink in light of the switch to
Kubernetes. (For example is the "environment" the same thing from an
infrastructure perspective as it is from the
cluster-user/developer/application perspective?)

The idea here is really just to define a place to put the common tags so
that we continue to set them in the right way. Using [provider default
tags] (relatively new feature) is now the cleanest way to do this.

Thanks to @kerin for the suggestion of using provider default tags.

One caveat is that the default tags aren't propagated to ASGs, so this
doesn't currently tag the node pool ASG. Passing the same set of tags to
the `eks` module in order to tag the ASG doesn't work, because the TF
provider unhelpfully forbids individual resources from overriding
provider-specific tags because of a design limitation of TF. (See
hashicorp/terraform-provider-aws#19204.)

[tagging guide]: https://github.com/alphagov/govuk-infrastructure/blob/main/terraform/docs/tagging-guide.md
[provider default tags]: https://www.hashicorp.com/blog/default-tags-in-the-terraform-aws-provider

terraform/deployments/cluster-infrastructure/main.tf

@@ -11,6 +11,13 @@ terraform {
 
 provider "aws" {
   region = "eu-west-1"
+  default_tags {
+    tags = {
+      project              = "replatforming"
+      repository           = "govuk-infrastructure"
+      terraform_deployment = basename(abspath(path.root))
+    }
+  }
 }
 
 module "eks" {