Import CSV backups

Atlassian is deprecating its Confluence XML backup format and now has a new one on Confluence Cloud: CSV.

Still a zip archive with an attachments folder, still basically a database dump, although in one CSV file per table, possibly gzipped, and the body contents are compressed with zstd and then encoded in base64 for site backups (but not space backups) for more fun. The dump is not a hibernate mapping anymore, it more directly matches the database tables.

We add initial support for this (that will need to be tested more before we can vet that it works well)

the confluence-xml module and the ConfluenceXMLPackage are now named less than ideally as they also handle CSV backups now, but both formats are still essentially the same thing and breaking compatibility with a rename would be a bit annoying.