cli: add a new 'backend' cli group to list and document backends
Adds a new 'backend' cli group with a 'list' command.
Example:
$ swh backend list vault
memory Stub vault backend, for use in the CLI.
postgresql Backend for the Software Heritage Vault.
remote Client to the Software Heritage vault cache.
$ swh backend list vault memory
vault:memory
Stub vault backend, for use in the CLI.
Edited by David Douard