[SPARK-3595] Respect configured OutputCommitters when calling saveAsHadoopFile

Addresses the issue in https://issues.apache.org/jira/browse/SPARK-3595, namely saveAsHadoopFile hardcoding the OutputCommitter.  This is not ideal when running Spark jobs that write to S3, especially when running them from an EMR cluster where the default OutputCommitter is a DirectOutputCommitter.
For this comment I'd make it more general:

```
// Use existing output committer if already set
```

I'm guessing over time we'll run into many formats that require this.