expose KeyValuePairs in `testkit.typed.LogginEvent`

[visma-alexander-maslov]

TestAppender.append(event: ILoggingEvent) creates LoggingEvent based on ch.qos.logback.classic.spi.ILoggingEvent

ILogginEvent has getKeyValuePairs

    /**
     * A list of {@link KeyValuePair} objects. The returned list may be null.
     * 
     * @return may be null
     * @since 1.3.0
     */
    List<KeyValuePair> getKeyValuePairs();

Would be nice to have it available as part of akka.actor.testkit.typed.LogginEvent.

It opens the possibility to use LoggingTestKit.withCustom(LoggingEvent => Boolean) for testing expected key->value(s) if needed.