[Github] Add support for Github App authentication

Our Github integration relies on a Personal Access Token (PAT) to authenticate against their API, however Github recommends using the Github app for authentication against their API:

_If you want to use the API on behalf of an organization or another user, GitHub recommends that you use a GitHub App. For more information, see "[About authentication with a GitHub App](https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/about-authentication-with-a-github-app)."

You can also create an OAuth token with an OAuth app to access the REST API. However, GitHub recommends that you use a GitHub App instead. GitHub Apps allow more control over the access and permission that the app has._

https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/about-authentication-to-github#authenticating-to-the-api-with-an-app

This issue will track our progress towards supporting Github App authentication.