Provide possibility to store large attachments in several ZIP files

[hagenw]

When a attaching a large file to the database or a folder containing lots of large files, audb will in both cases create a single very large ZIP file and try to upload it to the backend. This might fail for some backends, e.g. you can get a timeout when uploading large files to Artifactory depending on the server settings.

Maybe we could provide the option that a single ZIP file is used for each file in the attachment folder, or that a user could in general specify a max_archive_size in audb.config and the archive files are automatically split into smaller files before uploading.