move hot plate item placement stuff into its own system (#18923)

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

Content.Shared/Placeable/ItemPlacerComponent.cs

@@ -0,0 +1,31 @@
+using Content.Shared.Whitelist;
+using Robust.Shared.GameStates;
+
+namespace Content.Shared.Placeable;
+
+/// <summary>
+/// Detects items placed on it that match a whitelist.
+/// </summary>
+[RegisterComponent, NetworkedComponent, AutoGenerateComponentState, Access(typeof(ItemPlacerSystem))]
+public sealed partial class ItemPlacerComponent : Component
+{
+    /// <summary>
+    /// The entities that are currently on top of the placer.
+    /// Guaranteed to have less than <see cref="MaxEntities"/> enitites if it is set.
+    /// <summary>
+    [DataField("placedEntities"), AutoNetworkedField]
+    public HashSet<EntityUid> PlacedEntities = new();
+
+    /// <summary>
+    /// Whitelist for entities that can be placed.
+    /// </summary>
+    [DataField("whitelist"), ViewVariables(VVAccess.ReadWrite)]
+    public EntityWhitelist? Whitelist;
+
+    /// <summary>
+    /// The max amount of entities that can be placed at the same time.
+    /// If 0, there is no limit.
+    /// </summary>
+    [ViewVariables(VVAccess.ReadWrite), DataField("maxEntities"), AutoNetworkedField]
+    public uint MaxEntities = 1;
+}