Distributable session manager cannot detect immutable external classes

Currently, WildFly's distributed web session manager detects classes annotated with @Immutable placed into the web session, optimizes calls so HttpSession.getAttribute(...) are not treated as cache writes.

However, users don't always have access to the classes to be able to annotate them, e.g. they come from a shared library, web framework, etc.

Now that we have a distributed session management deployment descriptor, we have a convenient location to define external session attribute classes that should be treated as immutable.