[ASCollectionLayout] Add ASCollectionGalleryLayoutSizeProviding (#451)

* Add ASCollectionGalleryLayoutSizeProviding
- This allows users to return different sizes based on certain conditions, such as the collection node's bounds or grid constants.
- ASPagerNode will also act as a size provider to ensure all pages have an up-to-date size that is its bounds.

* Update CHANGELOG

* ASPagerNode to use gallery layout delegate if told to

Source/Details/ASCollectionGalleryLayoutDelegate.h

@@ -13,8 +13,25 @@
 #import <AsyncDisplayKit/ASCollectionLayoutDelegate.h>
 #import <AsyncDisplayKit/ASScrollDirection.h>
 
+@class ASElementMap;
+
 NS_ASSUME_NONNULL_BEGIN
 
+@protocol ASCollectionGalleryLayoutSizeProviding <NSObject>
+
+/**
+ * Returns the fixed size of each and every element.
+ *
+ * @discussion This method will only be called on main thread.
+ *
+ * @param elements All elements to be sized.
+ *
+ * @return The elements' size
+ */
+- (CGSize)sizeForElements:(ASElementMap *)elements;
+
+@end
+
 /**
  * A thread-safe layout delegate that arranges items with the same size into a flow layout.
  *
@@ -23,7 +40,9 @@ NS_ASSUME_NONNULL_BEGIN
 AS_SUBCLASSING_RESTRICTED
 @interface ASCollectionGalleryLayoutDelegate : NSObject <ASCollectionLayoutDelegate>
 
-- (instancetype)initWithScrollableDirections:(ASScrollDirection)scrollableDirections itemSize:(CGSize)itemSize NS_DESIGNATED_INITIALIZER;
+@property (nonatomic, weak) id<ASCollectionGalleryLayoutSizeProviding> sizeProvider;
+
+- (instancetype)initWithScrollableDirections:(ASScrollDirection)scrollableDirections NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)init __unavailable;