add support for attach in vpc private network

[remyleone]

Description

scw vpc private-network attach

First multiple choice asking for the type of resource to attach (instance, baremetal, rdb, load-balancer)
then a multiple choice asking among the type of resource in the given zone which one should be chosen.
Then the command attaches the given resource to the given private-network.

How I imagine scw could expose this functionality

References