Rework Update() in apstra/resource_freeform_resource.go

[chrismarget-j]

	// update the assignments if necessary
	if !plan.AssignedTo.Equal(state.AssignedTo) {
		var planAssignments []apstra.ObjectId
		resp.Diagnostics.Append(plan.AssignedTo.ElementsAs(ctx, &planAssignments, false)...)
		if resp.Diagnostics.HasError() {
			return
		}
		
		err = bp.UpdateResourceAssignments(ctx, apstra.ObjectId(plan.Id.ValueString()), planAssignments)
		if err != nil {
			resp.Diagnostics.AddError("error updating Resource Assignments", err.Error())
			return
		}
	}