[java-logging-logback] Logback encoders are not working with LoggingAppender

Logback encoders are not working with LoggingAppender. Can encoders be used with LoggingAppender for layout/pattern usage?

#### Environment details
Need to use https://logback.qos.ch/manual/encoders.html with google-cloud-logging-logback, but it doesn't work when I use encoder with google-cloud-logging-logback version 0.130.17-alpha.

#### Steps to reproduce

1. Use LoggingAppender in logback.xml
2. Add encoder element to logback.xml.

#### Code example

```
Contents of logback.xml:

DEBUG

application.log
gae_app
com.server.utils.LoggingEnhancerSimple
WARN

%d %-5level [%thread] %logger{0}: %msg%n

```

#### Stack trace
```
In App Engine Standard Java 17 log I see this error at startup:
`ERROR in ch.qos.logback.core.joran.spi.Interpreter@12:16 - no applicable action for [pattern], current ElementPath is [[configuration][appender][encoder][pattern]]`

```

#### External references such as API reference guides

https://github.com/googleapis/java-logging-logback
See "Usage" section which says encoders can be used with LoggingAppender.