Customizable musics

Currently, most of Cambridge's official game modes (including built-in) lacks the musics. In cambridge-stacker/cambridge#31, I made a feature request to add customizable music to Cambridge's official gamemodes.

The customizable musics are also meant to added to the official modpack's gamemodes, and requires the aforementioned issue from the main Cambridge repository to be completed.

Start by reviewing Cambridge issue #31 and the official modpack's gamemodes; no files, tests, or implementation entry points are named here. Confirm the upstream customizable-music feature is complete, then determine how each affected official gamemode should expose its music configuration and verify that all intended gamemodes have customizable music.

Written by the indexing model from the issue text.