Adds disposal mailing units (again) (#7630)

Content.Server/Disposal/Mailing/MailingUnitComponent.cs

@@ -0,0 +1,30 @@
+using Content.Shared.Disposal.Components;
+
+namespace Content.Server.Disposal.Mailing;
+
+[Access(typeof(MailingUnitSystem))]
+[RegisterComponent]
+public sealed class MailingUnitComponent : Component
+{
+    /// <summary>
+    /// List of targets the mailing unit can send to.
+    /// Each target is just a disposal routing tag
+    /// </summary>
+    [DataField("targetList")]
+    public readonly List<string> TargetList = new();
+
+    /// <summary>
+    /// The target that gets attached to the disposal holders tag list on flush
+    /// </summary>
+    [DataField("target")]
+    public string? Target;
+
+    /// <summary>
+    /// The tag for this mailing unit
+    /// </summary>
+    [ViewVariables(VVAccess.ReadWrite)]
+    [DataField("tag")]
+    public string? Tag;
+
+    public SharedDisposalUnitComponent.DisposalUnitBoundUserInterfaceState? DisposalUnitInterfaceState;
+}