Prevent mutatibility of schema properties

[rthery]

Even after reading the official documentation, I wasn't 100% certain that Colyseus was only server authoritative (although I was 95% maybe ;))

What made me doubt is the fact that schema properties are actually public fields in the generated C# classes!
Since state cannot be mutated directly on the client side, we shouldn't allow this. Best to make these fields internal with a public getter, the intent will then be clear for any C# developer.