Provide directive composition capabilities for Java DSL

[jlprat]

Currently Java DSL only offers route composition. In order to bring the Java DSL at par with the Scala DSL, it would be good to be able to apply composition at Directive level.
At the end it should be possible to write:

val route =
  path("order" / IntNumber) { id =>
    (get | put) { 
      complete(s"Received request for order $id")
    }
  }

like

return path(segment("order").slash(integerSegment()), id -> 
  orElse(Directives::get, Directives::put, () -> 
    complete("Received request for order " + id)
  )
)

[jlprat]

I could dive into this one as well 😄