Expose cacheExtent for ChatList

[Brandon-Wolff]

When there are several image messages in a row, scrolling back and forth causes them to reload even when the message is barely off screen. With the default image message builder, it causes a lot of jumping around as images load in. CachedNetworkImage will still display a progress indicator for a moment when the message enters the screen, but adding a sufficient cacheExtent value to the CustomScrollView fixes that.

[demchenkoalex]

Hey, current version is not optimised at all, and instead of adding properties I am working on a full re-design with the v2 version. Not jumping messages and cache will be in the core of the solution, making chat screen performant and smooth.

[demchenkoalex]

cache built-in in v2 #646