Add OpenTofu versions of validate and format

[rhoades-brown]

Description

I have started experimenting with OpenTofu and Terragrunt. Terragrunt supports OpenTofu natively, so I thought it would be good to add support for these tools.

Documentation

Updated the readme to mention the new hooks and added an example of how OpenTofu users can implement the git hooks.

TODOs

Please ensure all of these TODOs are completed before asking for a review.

• Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
• Update the docs.
• Keep the changes backward compatible where possible.
• Run the pre-commit checks successfully.
• Run the relevant tests successfully.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.