feat: Implement IRC Gateway Service and related functionality

- Add IrcGatewayService to handle IRC connections and commands.
- Create IrcMessage class for parsing IRC protocol lines.
- Implement IrcMessageFormatter for formatting messages as IRC lines.
- Define IrcNumericReply constants for IRC numeric replies.
- Add IrcOptions class for configuration settings related to IRC.
- Create IrcServiceExtensions for adding IRC services to the application.
- Refactor ChannelsController to use IChatService for broadcasting messages and channel updates.
- Update ChatHub to utilize IChatService for user connection and message handling.
- Introduce ChatService to manage chat-related operations and interactions.
- Implement SignalRBroadcaster for broadcasting messages to SignalR clients.
- Update PresenceTracker to retrieve usernames for connections.
- Modify appsettings.example.json to include IRC configuration options.
- Update solution file to include the new IRC project.

src/EchoHub.Server/Services/PresenceTracker.cs

@@ -137,6 +137,11 @@ public class PresenceTracker
         }
     }
 
+    public string? GetUsernameForConnection(string connectionId)
+    {
+        return _connections.TryGetValue(connectionId, out var info) ? info.username : null;
+    }
+
     public bool IsOnline(string username)
     {
         return _userConnections.TryGetValue(username, out var connections) && connections.Count > 0;