[Question] Is it possible to delay session_start event until I set a custom UserId?

### [REQUIRED] Please fill in the following fields:

* Unity editor version: 2020.3.17f1
* Firebase Unity SDK version: 17.0.0
* Source you installed the SDK: .unitypackage (.unitypackage or Unity Package Manager)
* Problematic Firebase Component: Analytics (Auth, Database, etc.)
* Other Firebase Components in use: Crashlytics (Auth, Database, etc.)
* Additional SDKs you are using: Not relevant (Facebook, AdMob, etc.)
* Platform you are using the Unity editor on: Windows (Mac, Windows, or Linux)
* Platform you are targeting: iOS, Android (iOS, Android, and/or desktop)
* Scripting Runtime: IL2CPP (Mono, and/or IL2CPP)

### [REQUIRED] Please describe the question here:
I would like to still initialize Crashlytics as soon as possible, but delay the very first session_start event until later when I get and can set a custom UserId