[Request] Separate "Duty BGM" level option

Basically I would like to have it so that I can have the background music when out in the world at one level (relatively quiet), but then as soon as I load into a duty like a dungeon/raid the plugin should automatically detect it and change the games BGM setting to my desired (higher) amount, then once I leave the instance and return to the outside world it should again detect it and automatically change it back to the original value again.

No files, tests, or entry points are named in the issue. Start by locating the plugin's existing volume-setting logic and the Dalamud entry point for detecting duty or zone transitions. Done means separate outside-world and duty BGM levels are applied automatically, with the outside-world value restored after leaving the instance.

Written by the indexing model from the issue text.