[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

examples/CustomCollectionView/Sample/MosaicCollectionLayoutDelegate.m

@@ -36,11 +36,13 @@
 
 - (ASScrollDirection)scrollableDirections
 {
+  ASDisplayNodeAssertMainThread();
   return ASScrollDirectionVerticalDirections;
 }
 
 - (id)additionalInfoForLayoutWithElements:(ASElementMap *)elements
 {
+  ASDisplayNodeAssertMainThread();
   return _info;
 }