Add terraform metadata graphql type

We added support to Integrate the Terraform README into the respons... (#438059 - closed) via app/presenters/packages/detail/package_presenter.rb which is enough to implement Add the Terraform README to the UI (#438060 - closed)

However this blocks Terraform registry UI to use GraphQL (#435850) as we now need to add GraphQL support for terraform metadata similar to https://gitlab.com/gitlab-org/gitlab/-/blob/0b3405f5c8db1f61fd91d53909aac4cd7d1c962e/app/graphql/types/packages/maven/metadatum_type.rb

This should allow the frontend to query the details like so:

package(id:"gid://gitlab/Packages::Package/285"){
    id
    __typename
    metadata {
      ... on TerraformMetadata {
        readme
      }
    }
  }
}