During tests of logback plugins is important to add have a way to safely test the encoder results.

Right now we can use `ch.qos.logback.core.read.ListAppender` to capture the generated log messages, however those only capture the events by themselves, not allowing for encoders and layouts to process the messages.

Here the suggestion is to make the `ListAppender` support for an `Encoder` attribute which will allow for `Layouts` and deeper elements to affect the message and allow for more complete tests.