Deprecate `item_assets` extension and move into collection core

[jsignell]

[tylanderson]

Should this be moved as a breaking change to v2.0 as it will require the item_assets extension to be removed completely in favor of collection core?

[gadomski]

Should this be moved as a breaking change to v2.0 as it will require the item_assets extension to be removed completely in favor of collection core?

The title of the issue is "deprecate", so we can do that safely in v1.12. Removal would come later, probably in v2 as you suggest.