Add a backups config abstraction.

This abstraction helps address the problem of hard-coded paths for various sources from which backups pull data.  It also provides a better encapsulation of the information that drives backups.
There is an existing pattern in the codebase which I think works well: use a "parameter struct" (BackupParams/BackupArgs). There's no behaviour attached to this. It's really just a poor man's **kwargs).