Way to specify directory for auto downloaded mods and plugins eg. CURSEFORGE_FILES

### Enhancement Type

Improve an existing feature

### Describe the enhancement

Asked around in the Discord server whether it is possible to specify directory for `CURSEFORGE_FILES` was told that it is not currently possible but could be enchanced and whether I could create a github issue for it. Discord message link to conversation if relevant: https://discord.com/channels/660567679458869252/660569641550217327/1384513328897851412

Original reasoning for this is that I wanted `CURSEFORGE_FILES` mods to download to `"/data/FeedTheBeast/mods"` rather than `"/data/mods"`. Reworded title to be more broad as I reckon this would also include other platforms where auto downloading content is possible. Thank you.

Trace the startup handling of CURSEFORGE_FILES and related auto-downloaded content settings; no source file or test is identified in the issue, so begin by locating that entry point. Done should allow the download directory to be specified for CURSEFORGE_FILES and other applicable platforms, including the requested /data/FeedTheBeast/mods path, with coverage for the behavior.

Written by the indexing model from the issue text.