Add getCacheTime(filename) to VaadinServlet (#11744)

This implementation by default adheres to the GWT Pristine Caching rules
and calculates 0 seconds for ".nocache." and 1 year for ".cache."
filenames. All other filenames will use the value configured in the
deployment configuration.
By exposing this to a method, developers can implement custom naming
schemes. Developers also can opt to set an expires header using this
value.

Change-Id: Ibc0d17d48d38bfa3bb28bdf3929ad314828be406