Suppress unchecked for generated adapter read & write methods

[jakekdodd]

Was going to put this on the constructor, but with the lazy initialization it belongs on the adapter's read and write methods.

Suppresses unchecked warnings for assignment to any TypeAdapter<T> where T itself is a generic type:

set__string_adapter = (TypeAdapter<Set<String>>) gson.getAdapter(TypeToken.getParameterized(Set.class, String.class));