Support custom filter in Kotlin DSL

[eleftherias]

Summary

We should add support for adding a custom filter in the Kotlin DSL.
The configuration would look like

@EnableWebSecurity
open class CustomFilterConfig : WebSecurityConfigurerAdapter() {
    override fun configure(http: HttpSecurity) {
        http {
            addFilterAt(CustomFilter(), UsernamePasswordAuthenticationFilter::class.java)
        }
    }
}