Create a DSL for registering and building handlers

[osoykan]

Option-1, sectioned

val mediator = mediator {
    pipelineBehaviours {
        MeasurePipelineBehaviour()
    }
    notificationHandlers {
        register<PingNotification, PingNotificationHandler>()
    }
    queryHandlers {
        register<PingQuery, PingQueryHandler>()
    }
    commandHandlers {
        register<PingCommand, PingCommandHandler>()
    }
}

Option-2, without section

val mediator = mediator {
    register<PingCommand, PingCommandHandler>()
    register<PingQuery, PingQueryHandler>()
    register<PingNotification, PingNotificationHandler>()
    register<MeasurePipelineBehaviour>()
}