Add AWS Terraform template

[clbarrell]

This PR drops in a CI template for applying AWS changes using Terraform.

It's mostly based on the workflow outlined in https://developer.hashicorp.com/terraform/tutorials/automation/automate-terraform

Another template could also be generalised down to terraform (minus AWS) if we wanted to removed OIDC part, which I think seems reasonable.

Here's an example:

[clbarrell]

Realised I already had made this one: https://github.com/buildkite/templates/pull/23