Add interlocking airlocks (#14177)

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/MachineLinking/Events/SignalReceivedEvent.cs

@@ -0,0 +1,23 @@
+namespace Content.Server.MachineLinking.Events
+{
+    public enum SignalState
+    {
+        Momentary, // Instantaneous pulse high, compatibility behavior
+        Low,
+        High
+    }
+
+    public sealed class SignalReceivedEvent : EntityEventArgs
+    {
+        public readonly string Port;
+        public readonly SignalState State;
+        public readonly EntityUid? Trigger;
+
+        public SignalReceivedEvent(string port, EntityUid? trigger, SignalState state)
+        {
+            Port = port;
+            Trigger = trigger;
+            State = state;
+        }
+    }
+}