Extract network resources into reusable modules/network module

Moves the three incus_network resources into a dedicated module called
once per entry in var.incus_networks. State was migrated imperatively
via `terraform state mv` because lxc/incus v1.0.2 does not implement
ResourceWithMoveState, making declarative moved blocks unsupported.
Plan confirms zero creates/destroys after migration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

incus/modules/network/outputs.tf

@@ -0,0 +1,7 @@
+output "name" {
+  value = incus_network.this.name
+}
+
+output "type" {
+  value = incus_network.this.type
+}