[FR] Make it easy to generate rawnonce and nonce for Sign in with Apple

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

* Unity editor version: 2019.4.26f1
* Firebase Unity SDK version: 7.0.2
* Source you installed the SDK: Unity Package
* Problematic Firebase Component: Auth
* Other Firebase Components in use: Auth, Database, Analytics,Cloud Functions
* Additional SDKs you are using: Facebook, Game Analytics
* Platform you are using the Unity editor on: Windows
* Platform you are targeting: IOS
* Scripting Runtime: IL2CPP

### [REQUIRED] Please describe the question here:

Hi there!

We are trying to provide multiple authentication options to our user such as Anonymous, Facebook, Google Play Game Services, Google Login, Game Center and Sign in with apple. I have worked around with almost all of them and succeed but Sign in with apple.

There are lack of resources about this topic and quickstart/firebase documents have not enough information. The question that i have is about the rawnonce.

I generate "rawnonce" as a random string which has 32 char length and i assume that i have to send it to firebase for authentication. There are also a nonce which is sha256 form of the rawnonce but i have no idea what to do with that. It says restore it on the app but even i restore it what if user deletes the app or clear the cache and what is the purpose of this string?

And if anyone of you have example of Siwa implementation it will be appreciated #145

Thankx.