fork setup-terraform into this repo

[imjasonh]

This is mainly a copy/paste of https://github.com/hashicorp/setup-terraform, on this new repo.

View the repo from this PR branch: https://github.com/imjasonh/setup-opentofu/tree/fork/

If you'd like to see a diff from the original repo, you can see that here: https://github.com/hashicorp/setup-terraform/compare/main...imjasonh:setup-terraform:fork

This is an early hacky attempt, and I'm sure I got stuff wrong. Tests pass, but I wouldn't bet production on it just yet. 😅

If there are other plans afoot to get this up and running I'll happily close this and follow that effort.

[imjasonh]

Looks like we'll also want to set up required CI checks before merging 😆