[Feature Request]: Python SDK | File IO | Dynamically update the parent directory.

### What would you like to happen?

We are looking into writing into separate directories using the python apache beam using fileio.WriteToFiles. But it only gives option to write into separate [files](https://github.com/apache/beam/blob/316c969b9c373177986ee0fbc794ed2887244328/sdks/python/apache_beam/io/fileio.py#L678-L679), not in separate parent directories.

We are writing into GCS (Google Cloud Storage) were we really don't have any directory/folder, it is a flat structure. So writing into a new file with different prefix doesn't require any folder creation.

I ran small tests by updating following inside `_MoveTempFilesIntoFinalDestinationFn` method -
```
final_full_path = filesystems.FileSystems.join(
self.path.get(), final_file_name)
```
to
```
final_full_path = final_file_name
```

Provided the whole path through `file_naming=`, it successfully wrote to two separate paths -
```
1. /bigstore/customer-A/dfasfasdfasdfas/...

2. /bigstore/customer-B/jlkjjoijmnyiouuy/...
```

We would like to update have this as part of the feature which would be beneficial while using the flat structure file systems like GCS.

### Issue Priority

Priority: 2

### Issue Component

Component: io-py-files