Sound Specifiers. (#4239)

* Adds SoundSpecifier and a type serializer.

* DiceComponent makes use of SoundSpecifier.

* When rider autorefactoring decices to add field:

* Remove SoundEmptySpecifier, use nullable everywhere

Content.Shared/Sound/SoundSpecifierTypeSerializer.cs

@@ -0,0 +1,48 @@
+using System;
+using Robust.Shared.IoC;
+using Robust.Shared.Serialization.Manager;
+using Robust.Shared.Serialization.Manager.Attributes;
+using Robust.Shared.Serialization.Manager.Result;
+using Robust.Shared.Serialization.Markdown.Mapping;
+using Robust.Shared.Serialization.Markdown.Validation;
+using Robust.Shared.Serialization.TypeSerializers.Interfaces;
+
+namespace Content.Shared.Sound
+{
+    [TypeSerializer]
+    public class SoundSpecifierTypeSerializer : ITypeReader<SoundSpecifier, MappingDataNode>
+    {
+        private Type GetType(MappingDataNode node)
+        {
+            var hasPath = node.Has(SoundPathSpecifier.Node);
+            var hasCollection = node.Has(SoundCollectionSpecifier.Node);
+
+            if (hasPath || !(hasPath ^ hasCollection))
+                return typeof(SoundPathSpecifier);
+
+            if (hasCollection)
+                return typeof(SoundCollectionSpecifier);
+
+            return typeof(SoundPathSpecifier);
+        }
+
+        public DeserializationResult Read(ISerializationManager serializationManager, MappingDataNode node,
+            IDependencyCollection dependencies, bool skipHook, ISerializationContext? context = null)
+        {
+            var type = GetType(node);
+            return serializationManager.Read(type, node, context, skipHook);
+        }
+
+        public ValidationNode Validate(ISerializationManager serializationManager, MappingDataNode node,
+            IDependencyCollection dependencies, ISerializationContext? context = null)
+        {
+            if (node.Has(SoundPathSpecifier.Node) && node.Has(SoundCollectionSpecifier.Node))
+                return new ErrorNode(node, "You can only specify either a sound path or a sound collection!");
+
+            if (!node.Has(SoundPathSpecifier.Node) && !node.Has(SoundCollectionSpecifier.Node))
+                return new ErrorNode(node, "You need to specify either a sound path or a sound collection!");
+
+            return serializationManager.ValidateNode(GetType(node), node, context);
+        }
+    }
+}