Map pool cvar (#12313)

* it just works

* nuke votable

* whoops

Content.Server/Maps/GameMapPoolPrototype.cs

@@ -0,0 +1,22 @@
+using JetBrains.Annotations;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototype.Set;
+
+namespace Content.Server.Maps;
+
+/// <summary>
+/// Prototype that holds a pool of maps that can be indexed based on the map pool CCVar.
+/// </summary>
+[Prototype("gameMapPool"), PublicAPI]
+public sealed class GameMapPoolPrototype : IPrototype
+{
+    /// <inheritdoc/>
+    [IdDataField]
+    public string ID { get; } = default!;
+
+    /// <summary>
+    ///     Which maps are in this pool.
+    /// </summary>
+    [DataField("maps", customTypeSerializer:typeof(PrototypeIdHashSetSerializer<GameMapPrototype>), required: true)]
+    public readonly HashSet<string> Maps = new(0);
+}