Use Kotlin DSL Marker Annotations to prevent scope leaking in WebFlux DSL

[eleftherias]

Summary

The Kotlin DSL is designed with a specific hierarchy in mind.
However, the hierarchy is not enforced.

The following configuration should not compile, but currently it does.

@EnableWebFluxSecurity
class SecurityConfig {

    @Bean
    fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
        return http {
            authorizeExchange {
                    authorizeExchange { }
            }
        }
    }
}

Kotlin allows the creation of DSL markers, to control the scope in which a function is available.

We can use a DSL marker to annotate all classes that belong to the server HTTP security DSL and prevent scope leaking.

For this we will need a new DSL marker, similar to @SecurityMarker.
A possible name for the new marker is @ServerSecurityMarker.

See fde3ccb for the changes that introduced the @SecurityMarker.