Improve parity between Java and Kotlin router DSL

[sdeleuze]

Add variants with pattern, predicate and handlerFunction to Kotlin router DSLs.
Equivalent of Builder GET(String pattern, RequestPredicate predicate, HandlerFunction<ServerResponse> handlerFunction); in Java.

[sdeleuze]

Merging #23526 in that one: Kotlin DSLs should contain add, filter, before, after and onError.