[java-logging-logback] Support jsonEncoder for structured logs with LoggingAppender

Seems like the structured logs using the jsonEncoder aren't supported when also using LoggingAppender.

Without supporting structured logs the log messages when ingesting into Cloud Logging lose useful metadata like severity, so filtering and searching is very limited. Using a LoggingEventEnchancer can be used to create structured logs with custom code, however, that seems that option should be used for advanced use cases. For the basic use cases it would be ideal to just configure the jsonEncoder.