Using maven-shade.plugin in elastic-apm-agent makes it impossible to customize log4j2 bytecode enhancement

## Is your feature request related to a problem?

apm-agent-java uses `log4j2` to logging, and the `maven-shade.plugin` is configured in the `elastic-apm-agent` module.
This will make it impossible to enhance the bytecode of log4j2 in customize apm-agent-plugins .
Can it be configured into the `apm-agent-core` module?
Or is there any way to make customized modules in `apm-agent-plugins` work properly when integrated into the agent?

for that:

```shell
apm-agent-plugins
├── README.md
├── ...
├── apm-customized-log4j2-plugin
```

## Describe the solution you'd like

Can it be configured into the `apm-agent-core` module?