Server-side backups: Add ability to configure object-storage part size

S3 multi-part uploads have a limit on the number of parts that can be used for a single upload (10000) and a configurable size of these parts (default 5MB). This means that with the default configuration the maximum file size of a backup bundle (or any backup file) is about 50GB. See https://docs.aws.amazon.com/AmazonS3/latest/userguide/qfacts.html

Since the part size is easily configurable in gocloud via https://pkg.go.dev/gocloud.dev/blob?utm_source=godoc#WriterOptions we should expose this as an option to users so that larger backups could be supported.