Enable HttpSession to be used outside of the scope of an HttpServletRequest

[markt-asf]

This is primarily intended to provide a mechanism to address jakartaee/websocket#175 but the solution is intended to be a general one.

There has been some prior discussion of this issue on the Servlet dev list:
https://www.eclipse.org/lists/servlet-dev/msg00566.html

This issue is to track the implementation of the solution that was broadly agreed on that thread.

I'll provide a PR along the lines of the solution discussed above. I expect there will be a few iterations to refine that solution before it is merged.