Refactor FirebaseUser to handle C++ objects better (#829)

* Refactor FirebaseUser to handle C++ objects better

* Update FirebaseUser.cs

* Add test

* Update readme

* Add comments to functions missing them

* Update auth.i

Author: a-maurice

auth/CMakeLists.txt

@@ -24,6 +24,7 @@ set(firebase_auth_swig
 # Firebase Auth CSharp files
 set(firebase_auth_src
   src/FirebaseAccountLinkException.cs
+  src/FirebaseUser.cs
   src/IUserInfo.cs
   src/PhoneAuthProvider.cs
 )