Add DTR entry with number of visible pairs (#52)

* Add DTR entry with number of visible pairs

* Simplify DTR visible pair counter

- Merge the new PairHandlerInvisibleMessage and PairHandlerVisibleDisposedMessage ;
- Leave alone a bunch of messages that were actually unneccessary.

* Match encodings with the rest of the codebase

* Make the DTR entry opt-in and add explanations

* More reliable publishing of PairHandlerVisible/Invisible

* Simplify things for DTR

* Rework async

MareSynchronos/PlayerData/Pairs/PairManager.cs

@@ -90,6 +90,8 @@ public sealed class PairManager : DisposableMediatorSubscriberBase
 
     public List<UserData> GetVisibleUsers() => _allClientPairs.Where(p => p.Value.IsVisible).Select(p => p.Key).ToList();
 
+    public int GetVisibleUserCount() => _allClientPairs.Count(p => p.Value.IsVisible);
+
     public void MarkPairOffline(UserData user)
     {
         if (_allClientPairs.TryGetValue(user, out var pair))