Handle nameservers as optional field in network (#521)

* make nameservers optional

* fix typo in unit test

* update docs

* Add UseStateForUnkown

---------

Co-authored-by: Vicente Pinto <vicente.pinto@freiheit.com>

stackit/internal/services/iaas/network/resource.go

@@ -10,6 +10,7 @@ import (
 	"github.com/hashicorp/terraform-plugin-framework/path"
 	"github.com/hashicorp/terraform-plugin-framework/resource"
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema"
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/listplanmodifier"
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema/stringplanmodifier"
 	"github.com/hashicorp/terraform-plugin-framework/schema/validator"
@@ -139,7 +140,11 @@ func (r *networkResource) Schema(_ context.Context, _ resource.SchemaRequest, re
 			},
 			"nameservers": schema.ListAttribute{
 				Description: "The nameservers of the network.",
-				Required:    true,
+				Optional:    true,
+				Computed:    true,
+				PlanModifiers: []planmodifier.List{
+					listplanmodifier.UseStateForUnknown(),
+				},
 				ElementType: types.StringType,
 			},
 			"ipv4_prefix_length": schema.Int64Attribute{

stackit/internal/services/iaas/network/resource_test.go

@@ -78,7 +78,7 @@ func TestMapFields(t *testing.T) {
 			true,
 		},
 		{
-			"nameservers_changed_outisde_tf",
+			"nameservers_changed_outside_tf",
 			Model{
 				ProjectId: types.StringValue("pid"),
 				NetworkId: types.StringValue("nid"),