GitHubApp should include slug

[ChadNedzlek]

This type https://github.com/octokit/octokit.net/blob/main/Octokit/Models/Response/GitHubApp.cs does not include the "slug" field documented here: https://docs.github.com/en/rest/reference/apps

Unfortunately, the "slug" field appears to be the name that appears with the app interacts with the repository (makes a comment or creates an issue), so we need access to that value for several scenarios.

[Whitestripe7773]

I would like to work on this one :)