savegame: migrate addiction variants

[andrei8l]

Summary

None

Purpose of change

Saved games with addiction events serialized before #56786 can't be loaded

Fixes: #56916

Describe the solution

Convert the addiction cata_variant on save load

Describe alternatives you've considered

N/A: events are lazily evaluated so the conversion needs to happen during I/O

Testing

1. Load this save Harrisonville.tar.gz
2. Observe the serialized event in #YW5kcmVp.sav :[ { "add_type": [ "add_type", "PKILLER" ], "character": [ "character_id", "1" ] }, { "count": 1, "first": 16415854, "last": 16415854 } ]
3. Quicksave and verify conversion to [ { "add_type": [ "addiction_id", "opiate" ], ... ]
4. Eat 100 poppy painkillers then quicksave
5. Confirm event count increased to 2

Additional context

Maybe there's a better way; let's see what dseguin or jbytheway have to say

[dseguin]

Thanks for fixing this! Looks good to go.