Node key backup assistance

[leewardbound]

Right now, we don't help backup node keys and expect our users to manage it themselves.

This is especially problematic when using leave-network, which blasts the sidechain directory.

I propose one of the following:

• Bare minimum, leave-network should drop keys into ~/.hydra/backups/{name}, possibly also including other relevant backup stuff (like config.toml or loom.yml)

• Alternatively/additionally, maybe it would be appropriate to consider adding something like hydra client save-identities --profile=my_node to backup the keys, and then a corresponding restore-identities option