Replace our own IExternalStorageHandler to abstract external inventores with the new IItemHandler to support any custom inventory as long as they provide their own IItemHandler implementation and without having them to explicitly implement and register a custom version just for AE2.

Note this requires a new way to handle internal network inventories exposed to storage buses through interfaces.

This also includes removing any other related class related to the IExternalStorageHandler like IExternalStorageRegistry and its implementation.

Fixed by #2508