Lobby Audio from static to CVar (#38375)

Lobby Audio from static to CVar (#38375)

* Changed LobbyMusiccollection over to a Cvar and edited ContentAudioSystem.cs to use Cvar Values

* Addedd Ability to modify the lobbyMusiccollection from the command line

* Fixed changing lobby music while in the round

* Deleted uneeded duplicate line

* Removed additional duplicate lobbyplaylist line

* Alphabatized imports and refactored to use Subs.CVar

* Added error checking and default behaviour to CVar sub.

* Refactored to use TryIndex and Allowed for a empty soundcollection when a sound collection is not found. Edited Cvar comment to reflect changes.

* Made _lobbyMusicCollection nullable and addedd handling for null case where used. Also Changed LobbyMusicCollection Cvar over to audio rather than ambience.

* Update Content.Server/Audio/ContentAudioSystem.cs

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Co-authored-by: Pieter-Jan Briers <pieterjan.briers@gmail.com>