feat(out_rdkafka2): adds `use_default_for_unknown_topic` configuration

[raytung]

Addresses https://github.com/fluent/fluent-plugin-kafka/issues/489

This lifts the same parameter from the out_kafka2 plugin and behaves similarly - it first attempts to write to a given topic, and if it fails because the topic does not exist, it will attempt to write to the default_topic

[ashie]

Thanks!