Expose the transmog data to Lua

Just wondering if there's any way for an addon to know if an item's appearance has been added to custom_unlocked_appearances ? I want to write an addon to include tooltip information when an item's appearance has already been unlocked.

The issue names custom_unlocked_appearances and a Lua addon tooltip use case, but it names no files, tests, or entry points. Start by locating the custom_unlocked_appearances handling and the module's Lua-facing interfaces; done means an addon can determine whether an item's appearance is already unlocked.

Written by the indexing model from the issue text.