Is your feature request related to a problem? Please describe.

The original idea was from @ilyagr in the Discord.

It would be nice if we could overlay different backends on each other, so we'd seamlessly support #2865 with Git and a Restic/Rustic backend.

Describe the solution you'd like

jj should support overlaying different backends on each other, so we'd only need to build a single backend which supports file/tree/user level commit redaction and ACL'd directories/files. This then could be overlayed on each storage backend which avoids the aformentioned requirement there.

Describe alternatives you've considered

Each large corporation is responsible for building a redaction backend by itself, which is I think is a net loss for the community.