Add backgroundView for ASCellNode #trivial (#1333)

Source/ASCellNode.h

@@ -194,6 +194,12 @@ typedef NS_ENUM(NSUInteger, ASCellNodeVisibilityEvent) {
  */
 @property (nullable) UIView *selectedBackgroundView;
 
+/* @abstract The view used as the background of the cell.
+ * ASTableView uses these properties when configuring UITableViewCells that host ASCellNodes.
+ * ASCollectionView uses these properties when configuring UICollectionViewCells that host ASCellNodes.
+ */
+@property (nullable) UIView *backgroundView;
+
 /* @abstract The accessory type view on the right side of the cell. Please take care of your ASLayoutSpec so that doesn't overlay the accessoryView
  * @default UITableViewCellAccessoryNone
  * ASTableView uses these properties when configuring UITableViewCells that host ASCellNodes.

Source/ASCollectionView.mm

@@ -1229,6 +1229,7 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
   // Update the selected background view in collectionView:willDisplayCell:forItemAtIndexPath: otherwise it could be too
   // early e.g. if the selectedBackgroundView was set in didLoad()
   cell.selectedBackgroundView = cellNode.selectedBackgroundView;
+  cell.backgroundView = cellNode.backgroundView;
   
   // Under iOS 10+, cells may be removed/re-added to the collection view without
   // receiving prepareForReuse/applyLayoutAttributes, as an optimization for e.g.

Source/ASTableView.mm

@@ -109,6 +109,7 @@ static NSString * const kCellReuseIdentifier = @"_ASTableViewCell";
   if (node) {
     self.backgroundColor = node.backgroundColor;
     self.selectedBackgroundView = node.selectedBackgroundView;
+    self.backgroundView = node.backgroundView;
 #if TARGET_OS_IOS
     self.separatorInset = node.separatorInset;
 #endif