feat: Store private data (Misc generic XML, private attributes) in Private state

[kklimonda-cl]

Is your feature request related to a problem?

#141 and #145 handle a problem where during Update() calls some of the resource data that is not managed
directly by terraform is lost.

That implementation unfortunately leads to multiple Read() and List() calls (one per resource) to get existing
data from the server, so it can be then used in Update() for fields not managed by terraform.

Describe the solution you'd like

A better implementation would make use of https://developer.hashicorp.com/terraform/plugin/framework/resources/private-state - during Read() and Create() calls we'd update private state for every managed resource, and then we'd use that during Update() instead of calling Read() again.