Add Spectator Mode to achievement settings

Spectator Mode is a new mode added by rc_client that allows for achievement and leaderboard functionality, but does not submit this data to the site, partially allowing for offline achievements. It effectively replaces the former settings for disabling achievements, leaderboards, and RP, which are now always active internally as long as the client is active.

Source/Core/Core/AchievementManager.h

@@ -105,6 +105,7 @@ public:
   std::recursive_mutex& GetLock();
   void SetHardcoreMode();
   bool IsHardcoreModeActive() const;
+  void SetSpectatorMode();
   std::string_view GetPlayerDisplayName() const;
   u32 GetPlayerScore() const;
   const BadgeStatus& GetPlayerBadge() const;