UseJava: Add RESOURCES with NAMESPACE to add_jar()

add_jar() currently requires (undocumented) that resources be supplied
as relative paths.  The resources *may* then end up in a path which does
not reflect the original path particularly when performing out-of-source
builds.  This change adds a RESOURCE (and NAMESPACE) parameter and a
function to add the names resources into the named namespace within the
jar- and thus address both of these problems.

Fixes: #22101

Tests/Java/HelloWorld.txt

@@ -0,0 +1 @@
+Hello World !