Add documentation and a command to see what are the fields that are going to be returned by a command

When I'm doing a command, I have 0 way to know in advance what are the fields that are going to be returned.
For instance, if I run `scw instance server list` I have no way to know what are the available property of a Server that is returned.
This is problematic because it is very difficult to use the templating feature for having custom output if I don't know how to get those fields.