Provide auto-configuration for WebFlux-based security

[hantsy]

When I created a Spring Boot 2.0.0.M3 project, I found the existing security starter did not include spring-secuirty-webflux. I had to add this artifact explicitly in pom.xml.

I think it is better to create a spring-boot-starter-security-webflux starter to add spring-security-webflux into project dependencies and take over the responsibility of @EnableWebFluxSecurity .

[wilkinsona]

This is already planned, although we don't appear to have a specific issue for it. We can use this one. Spring Security's WebFlux support is still a work in progress so this can't be implemented at the moment. We may be able to do this in M5, but it certainly won't be in M4.