New HttpSession method: getExpireTime()

Currently I hack into the servlet implementation (here `undertow`) to get the expire time. Custom calculations (like `lastAccessdTime + maxInactiveInterval`) are not really accurate and may not reflect all implementation specific changes to that time.
I need this for a customer requirement - show a notification X minutes before the session expires.

Maybe this timestamp could be added as a new API method named `getExpireTime`?

Start by reviewing the HttpSession API and the servlet specification, then examine how Undertow currently determines the session expiration time. Define the new method's semantics and update the relevant API documentation and compatibility tests; done means the expiration timestamp is exposed consistently without implementation-specific access.

Written by the indexing model from the issue text.