Validate some SpeciesPrototype fields (#35965)

* Convert SpeciesPrototype strings to ProtoIds

* Simplify protoman indexing calls

Content.Client/Humanoid/HumanoidAppearanceSystem.cs

@@ -58,7 +58,7 @@ public sealed class HumanoidAppearanceSystem : SharedHumanoidAppearanceSystem
 
         // add default species layers
         var speciesProto = _prototypeManager.Index(component.Species);
-        var baseSprites = _prototypeManager.Index<HumanoidSpeciesBaseSpritesPrototype>(speciesProto.SpriteSet);
+        var baseSprites = _prototypeManager.Index(speciesProto.SpriteSet);
         foreach (var (key, id) in baseSprites.Sprites)
         {
             oldLayers.Remove(key);