[ASCollectionNode] Add .collectionViewClass to ASCollectionNode+Beta. (#3198)

Although we may not want to support this property long-term, there are some usages of it
that are difficult to avoid. Specifically because the complexity it adds is both low,
and contained to a small area of the code that would be easy to remove it, it would be
great to support this.

The usage relates to apps that require the Interop protocol, and are architected to expect
a few methods / protocols being implemented on the UICollectionView class itself. It does
not directly override ASCollectionView behaviors. So hypothetically, it would also work
if it were possible to set ASCollectionView's superclass.

Instead, the app is making its own subclass descend from ASCollectionView and use the interop
APIs, even in environments where there are no ASCellNodes ever returned.

Source/ASCollectionNode+Beta.h

@@ -16,6 +16,13 @@ NS_ASSUME_NONNULL_BEGIN
 
 @interface ASCollectionNode (Beta)
 
+/**
+ * Allows providing a custom subclass of ASCollectionView to be managed by ASCollectionNode.
+ *
+ * @default [ASCollectionView class] is used whenever this property is unset or nil.
+ */
+@property (strong, nonatomic, nullable) Class collectionViewClass;
+
 - (instancetype)initWithFrame:(CGRect)frame collectionViewLayout:(UICollectionViewLayout *)layout layoutFacilitator:(nullable id<ASCollectionViewLayoutFacilitatorProtocol>)layoutFacilitator;
 
 - (void)beginUpdates ASDISPLAYNODE_DEPRECATED_MSG("Use -performBatchUpdates:completion: instead.");