Add support for repository variables

[cdav]

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 states that repository variables are supported but the implementation is missing. I see an open PR for adding repository variables.