We should probably set the log-call-back got from the properties before using it.

[leiwang008]

Get the log-call-back and set it to local field so that it can be used by the rd_kafka. We should also probably use the string constant instead of hardcoded string :-).

https://github.com/morganstanley/modern-cpp-kafka/blob/a146d10bcf166f55299c7a55728abaaea52cb0e5/include/kafka/KafkaClient.h#L383

    // Log Callback
    if (properties.contains(Config::LOG_CB))
    {
        setLogCallback(properties.get<LogCallback>(Config::LOG_CB));

        rd_kafka_conf_set_log_cb(rk_conf.get(), KafkaClient::logCallback);
    }

    // Statistics Callback
    if (properties.contains(Config::STATS_CB))
    {
        setStatsCallback(properties.get<StatsCallback>(Config::STATS_CB));

        rd_kafka_conf_set_stats_cb(rk_conf.get(), KafkaClient::statsCallback);
    }

    // Error Callback
    if (properties.contains(Config::ERROR_CB))
    {
        setErrorCallback(properties.get<ErrorCallback>(Config::ERROR_CB));

        rd_kafka_conf_set_error_cb(rk_conf.get(), KafkaClient::errorCallback);
    }

    // OAUTHBEARER Toker Refresh Callback
    if (properties.contains(Config::OAUTHBEARER_TOKEN_REFRESH_CB))
    {
        setOauthbearerTokenRefreshCallback(properties.get<OauthbearerTokenRefreshCallback>(Config::OAUTHBEARER_TOKEN_REFRESH_CB));

        rd_kafka_conf_set_oauthbearer_token_refresh_cb(rk_conf.get(), KafkaClient::oauthbearerTokenRefreshCallback);
    }

    // Interceptor
    if (properties.contains(Config::INTERCEPTORS))
    {
        setInterceptors(properties.get<Interceptors>(Config::INTERCEPTORS));

        const Error result{ rd_kafka_conf_interceptor_add_on_new(rk_conf.get(), "on_new", KafkaClient::configInterceptorOnNew, nullptr) };
        KAFKA_THROW_IF_WITH_ERROR(result);
    }