[RFC] Add a command to backup/restore Swarm state

In reaction to https://github.com/moby/moby/issues/33462, I was discussing with @diogomonica that if we consider the secrets store in swarm to be the "source of truth" (i.e., not just a "copy" of secrets stored/managed elsewhere), that there should be an easy way to backup and restore the swarm state, without users having to dive into /var/lib/docker/swarm to create an archive.

SwarmKit does have a tool to dump this data https://github.com/docker/swarmkit/blob/master/cmd/swarm-rafttool/dump.go, but AFAIK, this is not exposed through the docker CLI

Use-cases;

• backing up swarm state
• support (sending swarm state for investigation)
• migrating a cluster to new hardware / disaster recovery

Given that that directory contains both the data, and keys to decrypt the data, perhaps keys and data should be exported separately, so that data can be backed-up, and transported safely

/cc @aaronlehmann

Start with the SwarmKit swarm-rafttool/dump.go tool and the Docker CLI context described in the issue, then compare that with the data and keys under /var/lib/docker/swarm. Define the backup and restore command scope, including whether keys and data are exported separately. Done means users can back up and restore Swarm state without manually archiving that directory.