Add tofu steps to CI

[TobiasBabin]

This PR adds tofu init and tofu validate as additional steps to the standard CI for the TF modules in this GitHub org, supplementing the existing terraform steps.

The tofu commands are placed in a separate job so that code is checked out fresh and the .terraform files do not get in the way of each other.

The tofu test step is not added. [Edit] The tofu test step is included. Note: due to different behavior of OpenTofu when generating values in mocked providers, OpenTofu tests may fail until all generated values are explicitly mocked to adhere to the proper format. This is a known issue and kind of documented.

E.g. an AWS ARN value would have to be mocked like this:

mock_provider "aws" {
  mock_resource "aws_iam_policy" {
    defaults = {
      policy_arn = "arn:aws:iam::123456789012:policy/3c438f62b542dbe53234f3fd24d66a4fed329ac7"
    }
  }
}

Given that present modules are being backported to mock values in that way, the tofu test step is now included.