Add support for github_actions_variable

Feature request: Ability to create [GitHub repository variables](https://docs.github.com/en/actions/learn-github-actions/variables#creating-configuration-variables-for-a-repository)

It is supported in the underlying `integrations/github` module with the [github_actions_variable](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/actions_variable) resource.