Add shutdown attribute to Interfaces class and update related templates

- Added a `shutdown` boolean attribute to the `Interfaces` class to manage interface states.
- Updated the `eltex.ttp` template to include the `shutdown` command based on the new attribute.
- Modified expected configuration JSON files for various devices to reflect the new `shutdown` attribute, ensuring accurate testing and validation of interface configurations.

tests/fixtures/mikrotik/config.expected.json

@@ -7,24 +7,28 @@
   "interfaces": [
     {
       "interface": "bridge.LAN",
+      "shutdown": false,
       "ip_address": "172.16.3.1",
       "mask": 24,
       "description": null
     },
     {
       "interface": "ether1",
+      "shutdown": false,
       "ip_address": "10.38.3.245",
       "mask": 24,
       "description": null
     },
     {
       "interface": "wireguard2",
+      "shutdown": false,
       "ip_address": "10.1.100.2",
       "mask": 24,
       "description": null
     },
     {
       "interface": "ether4",
+      "shutdown": false,
       "ip_address": "100.10.10.1",
       "mask": 24,
       "description": null