Previously, there were a number of inconsistencies in the embedded
containers' handling of static resources. The Servlet spec requires
that static resources can be served from the META-INF/resources/
directory of jars nested inside a war in WEB-INF/lib/. The intention
was also to extend this to cover jar packaging when jars are nested in
BOOT-INF/lib/. This worked when using Tomcat as long as Jasper was on
the classpath. If you didn't have Jasper on the classpath or you
were using Jetty or Undertow it did not work.

This commit updates the configuration of embedded Jetty, Tomcat, and
Undertow so that all three containers handle static resources in the
same way, serving them from jars in WEB-INF/lib/ or /BOOT-INF/lib/.
Numerous intergration tests have been added to verify the behaviour,
including tests for Tomcat 8.0 and 7.0 which is supported in addition
to the default 8.5.x. Note that static resource handling only works
with Jetty 9.3.x and 9.2 and earlier does not support nested jars (
see https://github.com/eclipse/jetty.project/issues/518 for details).

Closes gh-8299