[Github] Add support for Github App authentication

[jamiehynds]

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."

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.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)