Remove Newtosoft and make it AOT compatible

[gajo357]

FirebaseAdmin is not compatible with .NET 10 AOT.

You cannot initialize GoogleCredential, and VerifyIdTokenAsync also fails.

For example

Newtonsoft.Json.JsonSerializationException: Unable to find a constructor to use for type FirebaseAdmin.Auth.FirebaseToken+Args. A class should either have a default constructor, one constructor with arguments or a constructor marked with the JsonConstructor attribute. Path 'IsAdmin', line 1, position 11.

[praneetnadkar]

Other than the code itself, its the using Google.Apis.Util and Google.Apis.Core that use newtonsoft. The serialization boundaries need to be identified and gradual updates would be needed I think