Add Galoshes slowdown over slippery surfaces (#30967)

* first draft

* Fixed it all, just need to rename stuff

* Rename and add comments

* Clean-up

* Access added

Content.Shared/Slippery/GetSlowedOverSlipperyModifierEvent.cs

@@ -0,0 +1,10 @@
+using Content.Shared.Inventory;
+
+namespace Content.Shared.Slippery;
+[ByRefEvent]
+public record struct GetSlowedOverSlipperyModifierEvent() : IInventoryRelayEvent
+{
+    SlotFlags IInventoryRelayEvent.TargetSlots => ~SlotFlags.POCKET;
+
+    public float SlowdownModifier = 1f;
+}