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 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
• as Github App installation
• as Github App 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.Clients, 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.Clients in total.

Inizia esaminando github.Client, AppServiceCreateInstallationToken e ChecksService, insieme ai tre flussi di autenticazione di GitHub collegati nell’issue. Definisci il supporto first-party necessario per le credenziali dell’app, dell’installazione e dell’utente, inclusi il riutilizzo sicuro e il rinnovo dei token di breve durata tra le chiamate API.