Expose distance spacing in IPositionSnapProvider for updating distance grid

Alternate method is to expose a `SnapDistancesChanged` event in
`IPositionSnapProvider` instead, but I chose this way as an analogue to
`IBeatSnapProvider.BeatDivisor`, which might even make sense to be
exposed as `BindableBeatDivisor` instead of caching that separately.

osu.Game/Rulesets/Edit/HitObjectComposer.cs

@@ -563,6 +563,8 @@ namespace osu.Game.Rulesets.Edit
 
         #region IPositionSnapProvider
 
+        public abstract IBindable<float> DistanceSpacingMultiplier { get; }
+
         public abstract SnapResult SnapScreenSpacePositionToValidTime(Vector2 screenSpacePosition);
 
         public virtual SnapResult SnapScreenSpacePositionToValidPosition(Vector2 screenSpacePosition) =>