feat: add steam_disk_space egg feature to SteamCMD eggs

A new egg feature modal was introduced in Panel v1.7.0 triggering when server has run out of disk space to provide more user-friendly instructions to both admins and normal users. This is unique to SteamCMD specific errors.

game_eggs/steamcmd_servers/left4dead_2/egg-left4dead_2.json

@@ -8,7 +8,9 @@
     "name": "Left 4 Dead 2",
     "author": "pterodactyl@mazoyer.eu",
     "description": "Left 4 Dead 2 is set in the aftermath of a worldwide pandemic of a disease nicknamed the \"Green Flu\", which rapidly transforms humans into zombie-like creatures and mutated forms that demonstrate extreme aggression towards non-infected beings. A few humans are immune to the disease, while some of those who are infected have no symptoms. The Civil Emergency and Defense Agency (CEDA) and the U.S. military create safe zones to attempt to evacuate as many survivors as possible.",
-    "features": null,
+    "features": [
+        "steam_disk_space"
+    ],
     "images": [
         "ghcr.io\/parkervcp\/games:source"
     ],