Add support for repository variables

## Prerequisites:

* Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.


* Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).

https://docs.github.com/en/rest/actions/variables?apiVersion=2022-11-28#update-a-repository-variable

## New Feature

Please describe the desired new functionality:

This request is part Bug report, part feature request. The [documentation](https://github.com/github/safe-settings/blob/main-enterprise/docs/github-settings/2.%20repository-variables.md) states that repository variables are supported but the implementation is missing. I see an [open PR](https://github.com/github/safe-settings/pull/635) for adding repository variables.