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/variables.tf

@@ -0,0 +1,26 @@
+variable "name" {
+  type = string
+}
+
+variable "type" {
+  type    = string
+  default = "physical"
+}
+
+variable "hypervisors" {
+  type = set(string)
+}
+
+variable "witnesses" {
+  type = set(string)
+}
+
+variable "hypervisor_parent" {
+  type    = string
+  default = "br0"
+}
+
+variable "witness_parent" {
+  type    = string
+  default = "dummy0"
+}