Add first party support for Github App

Many of the apis requires to be invoked as Github App, eg. https://docs.github.com/en/rest/checks/runs?apiVersion=2022-11-28. And currently we need to use third-party packages like [ghinstallation](https://github.com/bradleyfalzon/ghinstallation) to do so.

However, this is not the full story. When creating a Github App, it involves many different kinds of creds:

- [as Github App itself](https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-as-a-github-app)
- [as Github App installation](https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-as-a-github-app-installation)
- [as Github App user](https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-with-a-github-app-on-behalf-of-a-user)

And we need to use different cred to call different APIs. For now, the only way to do so is to create many different `github.Client`s, each holding a different credential.

And we need to do this with extra care, as all these token are short-lived, and need to be cached/refreshed periodically. Which means, blindly creating those clients will cause lots of rate limiting issues.

For instance, to use `github.ChecksService`, we need:

- Auth as Github App itself by creating a JWT with app's private key, to call `github.AppServiceCreateInstallationToken`
- Once we have the token, we can auth as app installation, then calling `github.ChecksService`

Which requires 2 `github.Client`s in total.