Includeresource duplicate strategy to append to existing file when unrolling jar

[chrisrueger]

Closes #6325

Adds a duplicate strategy to -includeresource (similar to :flatten or :rename)

duplicates:=APPEND

@${repo;org.apache.xmlgraphics:fop-core;latest}!/META-INF/services/*
@${repo;org.apache.xmlgraphics:xmlgraphics-commons;latest}!/META-INF/services/*;:duplicates:=APPEND

@pkriens This is a first draft based on our discussion last week.