Support Spring WebFlux

[flowersinthesand]

Spring WebFlux is a web framework of Spring's reactive stack based on Reactive Streams:

The reactive stack, web framework, Spring WebFlux, was added later in version 5.0. It is fully non-blocking, supports Reactive Streams back pressure, and runs on servers such as Netty, Undertow, and Servlet 3.1+ containers.

See https://docs.spring.io/spring/docs/5.0.6.RELEASE/spring-framework-reference/web-reactive.html