Add Kotlin DSL for functional routes

[jonasws]

This PR adds a DSL matching that of org.springframework.web.reactive.function.server.RouterFunctionDsl to allow users to define operations inline when using functional routes, like (imports omitted for brevity):

@Configuration
class RouterConfig {
    @Bean
    fun appRouter(clientHandler: ClientHandler) = docRouter {
        GET(
            "/clients//{clientName}",
            clientHandler::getClientByName
        ) {
            it.beanClass(ClientService::class.java).beanMethod("getClient")
                .parameter(Builder.parameterBuilder().name("clientName"))
        }
    }
}

This has been adressed earlier as well, in #967

[bnasslahsen]

Thank you @jonasws for your contribution.