add gateway for admeme (#17587)

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Shared/Gateway/GatewayUi.cs

@@ -0,0 +1,65 @@
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.Gateway;
+
+[Serializable, NetSerializable]
+public enum GatewayVisuals : byte
+{
+    Active
+}
+
+[Serializable, NetSerializable]
+public enum GatewayVisualLayers : byte
+{
+    Portal
+}
+
+[Serializable, NetSerializable]
+public enum GatewayUiKey : byte
+{
+    Key
+}
+
+[Serializable, NetSerializable]
+public sealed class GatewayBoundUserInterfaceState : BoundUserInterfaceState
+{
+    /// <summary>
+    /// List of enabled destinations and information about them.
+    /// </summary>
+    public readonly List<(EntityUid, string, TimeSpan, bool)> Destinations;
+
+    /// <summary>
+    /// Which destination it is currently linked to, if any.
+    /// </summary>
+    public readonly EntityUid? Current;
+
+    /// <summary>
+    /// Time the portal will close at.
+    /// </summary>
+    public readonly TimeSpan NextClose;
+
+    /// <summary>
+    /// Time the portal last opened at.
+    /// </summary>
+    public readonly TimeSpan LastOpen;
+
+    public GatewayBoundUserInterfaceState(List<(EntityUid, string, TimeSpan, bool)> destinations,
+        EntityUid? current, TimeSpan nextClose, TimeSpan lastOpen)
+    {
+        Destinations = destinations;
+        Current = current;
+        NextClose = nextClose;
+        LastOpen = lastOpen;
+    }
+}
+
+[Serializable, NetSerializable]
+public sealed class GatewayOpenPortalMessage : BoundUserInterfaceMessage
+{
+    public EntityUid Destination;
+
+    public GatewayOpenPortalMessage(EntityUid destination)
+    {
+        Destination = destination;
+    }
+}