Add entity list prototype (#3720)

* Add entity list prototype

* Turn entities property into a method

Content.Shared/Prototypes/EntityList/EntityListPrototype.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using Robust.Shared.IoC;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization.Manager.Attributes;
+using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototype.List;
+using Robust.Shared.ViewVariables;
+
+namespace Content.Shared.Prototypes.EntityList
+{
+    [Prototype("entityList")]
+    public class EntityListPrototype : IPrototype
+    {
+        [ViewVariables]
+        [field: DataField("id", required: true)]
+        public string ID { get; } = default!;
+
+        [ViewVariables]
+        [field: DataField("entities", customTypeSerializer: typeof(PrototypeIdListSerializer<EntityPrototype>))]
+        public ImmutableList<string> EntityIds { get; } = ImmutableList<string>.Empty;
+
+        public IEnumerable<EntityPrototype> Entities(IPrototypeManager? prototypeManager = null)
+        {
+            prototypeManager ??= IoCManager.Resolve<IPrototypeManager>();
+
+            foreach (var entityId in EntityIds)
+            {
+                yield return prototypeManager.Index<EntityPrototype>(entityId);
+            }
+        }
+    }
+}