[FR] Add isNewUser property to AdditionalUserInfo

[mdn0420]

Feature proposal

• Firebase Component: Auth

Add the AdditionalUserInfo::isNewUser property. It exists in the SDKs for other platforms but not Unity's.

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[a-maurice]

Thanks for calling this out. We will need to add it to the C++ repo, and figure out how to get the information on the desktop implementation for the Unity editor support, so it might take a while, but we will look into it.