add commands for listing and inspecting instance-setup scripts

It would be nice to have the following additional commands:

- `cloudy instance-setup list`

List all available instance-setup scripts.

Note that this more-or-less already exists in the output of the `cloudy scaleway create --help` command.

- `cloudy instance-setup show INSTANCE_SETUP_SCRIPT`

Show the raw YAML contents of the `INSTANCE_SETUP_SCRIPT`. The possible values for `INSTANCE_SETUP_SCRIPT` should come from the output of the `cloudy instance-setup list` command.

- `cloudy instance-info` (or `cloudy info`, or `cloudy show`)

Show detailed output about a single instance, including both cloud agnostic and cloud-specific details. This should also show the full output of the cloud-init scripts specified when creating the instance.