Allow to set default installationId in strategy Options about auth-app.js

[gr2m]

I’ll put that on hold for now and wait until we made sure that this is indeed helpful for Probot and similar frameworks. I could see it being confusing that the same method call auth() can return different things based on the factory options.

[gr2m]

const auth = createAppAuth({
  id: 1,
  privateKey: "-----BEGIN RSA PRIVATE KEY-----\n...",
  installationId: 123
});

new Octokit({
  auth
});

The auth option for the Octokit constructor would retrieve the current request options which includes the .url property (this is not implemented that way, but that’s what I plan to do). So the correct authentication method would be used automatically based on the endpoint used 🤔 That’s pretty neat.

[gr2m]

that's done