Make use of RetriableException

In order to make use of the kafka connector retrying mechanism, would it be possible to throw a [RetriableException](https://kafka.apache.org/0100/javadoc/index.html?org/apache/kafka/common/errors/RetriableException.html) instead of a ConnectException [here](https://github.com/aiven/http-connector-for-apache-kafka/blob/main/src/main/java/io/aiven/kafka/connect/http/sender/HttpSender.java#L96) and [here](https://github.com/aiven/http-connector-for-apache-kafka/blob/main/src/main/java/io/aiven/kafka/connect/http/sender/HttpSender.java#L100). This would enable us to make use of these configuration options.

- [errors.retry.timeout](https://docs.confluent.io/platform/current/installation/configuration/connect/sink-connect-configs.html#sinkconnectorconfigs_errors.retry.timeout)
- [errors.retry.delay.max.ms](https://docs.confluent.io/platform/current/installation/configuration/connect/sink-connect-configs.html#sinkconnectorconfigs_errors.retry.delay.max.ms)

The functionality that would be nice to have is that after the connector fails (after `max.retries` is exhausted) is that kafka automatically retries on set intervals to bring the connector back.