Add tfplan conversion script to Terraform.gitlab-ci.yml template

Goal

Create something easily consumable by the exsting artifact process that does not include secrets to prevent leakage.

Definition of Done

• Work with whoever works on #207527 (closed) to come up with a file format
• Script should convert the tfplan to a consumable json file
• json should only include information needed to display in mr widget
• json file should not include secrets
• script should live in https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Terraform.gitlab-ci.yml

Notes

• We do this in other autodevops templates, take a look at them to see examples.