starts moving alarm sync logic to alarmables

Content.Server/Atmos/Monitor/Components/AtmosAlarmingComponent.cs

@@ -0,0 +1,14 @@
+namespace Content.Server.Atmos.Monitor.Components;
+
+[RegisterComponent]
+public sealed class AtmosAlarmingComponent : Component
+{
+    /// <summary>
+    ///     All registered receivers in this alarmer.
+    /// </summary>
+    public HashSet<string> RegisteredReceivers = new();
+
+    // Somebody should do this someday. I'll leave it here as a reminder,
+    // just in case.
+    // public string StationAlarmMonitorFrequencyId
+}