[Close #159] feat: support github action variables

[volker-raschek]

This PR add support of GitHub action variables requested in #159.

Please review my changes:

1. The pre-commit throws actually an issue with the latest version of the master branch. This patch will not existing issues.
2. I've executed the go unittests locally. Some tests throws an error, because instead of an organization I used my personal namespace (account).
3. I added the resource github_actions_variable also into the secrets.tf file, because based on the naming pattern would this leads to irritations, because the file variables.tf already exists.
4. Requires integrations/github >= v6.0