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.