[Feature Request]: Support slf4j MDC within DataflowWorkerLoggingHandler

### What would you like to happen?

Since the Dataflow documentation suggests to use Slf4j for logging it would only make sense that Dataflow would also log the `org.slf4j.MDC` key value pairs into the JSON logging object.

This could look something like this:
```java
Map mdc = MDC.getCopyOfContextMap();
if(!mdc.isEmpty() {
generator.writeFieldName("mdc");
generator.writeStartObject();
for(Entry entry : mdc.entrySet()) {
generator.writeStringField(entry.getKey(), entry.getValue());
}
generator.writeEndObject();
}
```

If this is an accepted feature I would be willing to work on this.

### Issue Priority

Priority: 2

### Issue Component

Component: runner-dataflow