Add githubrun package

* parses the event payload which exists at GITHUB_EVENT_PATH and the
corresponding struct type will be returned
https://help.github.com/en/actions/reference/events-that-trigger-workflows
* parses the default environment variables
https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables
* parses GITHUB_TOKEN if supplied
https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token