fix(instance): server: fix deletions in private_network list

[Mia-Cross]

We noticed unexpected behaviors in the private_network list attribute of the scaleway_instance_server resource.

• When we switch the order of 2 pn_ids in the list, both get removed.
• When deleting 2 pn_ids out of a list of 4, only 1 PN remains.

Notes:

• For now, I attempted to turn the TypeList into a TypeSet, which solved the first part of the test (the ordering problem), but introduces phantom private networks (elements with only the computed attributes set, but pn_id empty) in the second part: when deleting 2 pn_ids out of 4, there are actually 3 PNs that are read from the user config by Terraform (including 1 phantom pn), then 4 again (2 phantoms) when checking resource attributes values.
• Using sets is tricky for this attribute because, originally, the hash is calculated with all attributes of the element, which means that, on create, the element's hash will be based only on pn_id, but when reading the computed attributes, the element has new values inside and the hash changes. I attempted to avoid this by having a custom hash func that only considers the pn_id, but this seems to be what's generating the phantom elements.