Develop a Terraform CI/CD template for the GKE gitlab runner

Alongside https://gitlab.developers.cam.ac.uk/uis/devops/devhub/gitlab-runner-infrastructure/-/issues/2 we should develop an end-to-end terraform deployment pipeline template specifically for use with this new GKE gitlab runner.

It should contain the following:

• Test jobs
  • terraform validate and fmt
  • tflint
  • tfsec
• Plan jobs
  • Plans should only run when a MR is open.
  • Plans should be run against every environment and MR reports should be generated for the Gitlab UI.
• Apply jobs
  • Apply jobs should only run on. commits/merges to the default branch.
  • Apply to staging environment should always happen automatically.
  • All other envs should require manual trigger for apply.