Terraform plan before apply

[devopsmatt]

The TF GHA currently uses terraform-stats to produce drift output.
We want to see the terraform plan output before terraform apply is run.

To get there, we want to add a step that runs terraform plan -out=tf.plan here

Then modify the step that runs terraform apply to use the tf.plan plan file.

Bonus, optional - the plan is sent to Slack's notifications channel

DoD: PR with updated github actions in the repo.