Move Terraform APIs into OpenAPI namespace

What does this MR do and why?

Moves Terraform APIs into OpenAPI namespace, including:

• Adding description and detail where missing
• Adding success/failure codes
• Grouping under terraform_state and terraform_registry tags

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/376997, https://gitlab.com/gitlab-org/gitlab/-/issues/376999, https://gitlab.com/gitlab-org/gitlab/-/issues/376998