Make FlatFileItemReader and FlatFileItemWriter and its builders more reusable/abstract.

Hi,

I was wondering if there's a way to make FlatFileItemReader and FlatFileItemWriter and its builder implementations more abstract or reusable.

I was creating a SftpFlatFileItemReader and Writer for reading and writing direclty to the SFTP Server, and I encountered that the FlatFileItemWriter implementation is really attached to the definition of 'File' as in a FileSystem file.

On the other hand, the SftpFlatFileItemReader implementation was way easier since it accepts an InputStream as a Resource, and the only thing I had to override was the open and close methods, to manage the SFTP Server session.

But like I said, there's no way to write directly to a SFTP Server file with the current FlatFileItemWriter implementation with only the modifications I did for the SftpFlatFileItemReader.

Also, in order to create a SftpFlatFileItemReaderBuilder and WriterBuilder, I had to copy-paste many of the code that really has the same functionality in both environments, local FileSystem and SFTP, and of course, this code could change in Spring, and it's another level of complexity for us, since we would have to be "leveled" with this implementation as time goes and the classes changes.

So the issue splits in two:

1. Is there a way to make FlatFileItemWriter accept a Resource that is not a local File, but an abstract 'Resource' as in the FlatFileItemReader, which is more "abstract"?
2. Would it be possible to abstract the builders so there's no need to re-implement all the methods by copy-pasting?

Sorry for the inconvenience and thanks in advance 😄

Review the FlatFileItemReader and FlatFileItemWriter implementations alongside their builder implementations, then compare the existing SftpFlatFileItemReader approach described in the issue. Determine whether the reader and writer can share reusable Resource-based behavior and builders without copy-pasted functionality; done means the requested SFTP use case can be supported by the shared abstractions.