Description
We currently store the key (ssl.key.password), keystore (ssl.keystore.password) and truststore (ssl.truststore.password) passwords as a String in `KafkaConfig`, `ConsumerConfig` and `ProducerConfig`.
The problem with this approach is that we may accidentally log the password when logging the config.
A possible solution is to introduce a new `ConfigDef.Type` that overrides `toString` so that the value is hidden.