Make api keys optional

[DavidDTA]

API keys are not required when using OAuth, and I believe providing one should not be required to create a Firestore.Config value.

[IzumiSy]

In OAuth, Firebase REST API requires us to pass OAuth 2.0 token on Authorization header as the doc says.
https://firebase.google.com/docs/firestore/use-rest-api

So it should be required.

[DavidDTA]

The OAuth 2.0 token is passed via withAuthorization. However, in order to create a Config value, you must use Config.new, which requires an apiKey. The apiKey should be optional since it's not needed when withAuthorization is used.