There are 2 parts to this:

The first, is an addition to the existing API, to address a situation encountered in MDL-67585. This situation is when a component wishes to store favourites for an area using multiple different itemtype strings. In such a case, we'd like to be able to get the favourites for all the item types at once - at present, this must be done via multiple API calls to find_favourites_by_type.

The second is to update the calling code in content_item_readonly_repository, to use the single new API call instead of several calls in a loop (the current situation).