Add support for Backblaze B2
Hello,
This merge request adds support for backing up to Backblaze B2 cloud storage. This feature was previously requested in Bug #1523520.
At the moment the option is hidden by default like S3 and the other cloud storage engines. I think this is a decent compromise as it's available for the users that need it but not visible by default.
I was able to successfully create multiple backups and restore files.
This is my first time writing Vala or contributing to a Gnome project so let me know if anything looks off.
Thanks!