network: Support static subnet

roles/network/defaults/main.yaml

@@ -1,2 +1,3 @@
 ---
 network_driver: bridge
+network_subnet: ""

roles/network/meta/argument_specs.yaml

@@ -15,3 +15,8 @@ argument_specs:
         choices:
           - bridge
           - macvlan
+      network_subnet:
+        description: Subnet for the network
+        type: str
+        required: false
+        default: ""

roles/network/tasks/main.yaml

@@ -5,6 +5,7 @@
     state: quadlet
     quadlet_file_mode: "0644"
     driver: "{{ network_driver }}"
+    subnet: "{{ network_subnet if network_subnet | length > 0 else omit }}"
     quadlet_options: >-
       {{
         ['Options=parent=' ~ ansible_default_ipv4.interface]