Enhancement: Make `key` command's actions separate sub-commands

Each of the `add`, `list`, `remove` and `passwd` actions provided by the `key`
command is now a separate sub-command and have its own documentation which can
be invoked using `restic key <add|list|remove|passwd> --help`.

https://github.com/restic/restic/issues/4676
https://github.com/restic/restic/pull/4685
