parquet: add method to get both the inner writer and the file metadata when closing SerializedFileWriter

[conradludgate]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

I want to access the FileMetadata from a closed parquet file so that I can add some logging, but I also need to access the inner writer for further processing.

Describe the solution you'd like

SerializedFileWriter offers

• into_inner() -> Result<W>
• close() -> Result<FileMetadata>.

The bodies of both functions are almost identical. Perhaps close can return Result<(FileMetadata, W)>.

Describe alternatives you've considered

For now, I will use into_inner() and then open the file with SerializedFileReader to get the metadata.

Additional context

1. close() does not flush the file, which will ignore errors.
2. I would like async support, but I don't want to go through arrow. For now I am writing to an in memory buffer and then flushing the buffer over the network after I close the file.
3. TrackedWriter forces a BufWriter which means I am now double buffered. I would prefer if W: BufWriter instead of forcing the buffer on top.

[tustvold]

Adding a finish method that returns both makes sense to me

[tustvold]

label_issue.py automatically added labels {'parquet'} from #5471