[ADD] Filter: pre-process incoming data and post-process outgoing data

Filters are part of a filter chain, thus following the chain of responsability
design pattern.
More information are available in library/server/wsf/src/filter/README.md

examples/filter/readme.md

@@ -0,0 +1,4 @@
+Filter example
+
+To test the example, you can just run in a terminal:
+> curl -u foo:bar http://localhost:9090/user/1 -v