Add Kotlin example showing integration with WebTestClient

[eleftherias]

In Java code a WebTestClient can be configured as follows

WebTestClient
    .bindToApplicationContext(this.context)
    // add Spring Security test Support
    .apply(springSecurity())
    .configureClient()
    .filter(basicAuthentication())
    .build();

In Kotlin, it is not currently possible to use the apply method.
See spring-projects/spring-framework#20606 for more details and a workaround.

Once this issue is fixed, we should update the reference documentation to show the equivalent example in Kotlin.