add !mp addref <username> command

[ILW8]

Referees can execute commands on the behalf of the host running this client. The referee list is local and ephemeral, it last only as long as the user running this client stays in the lobby and the list of referees is not preserved when passing host to another user.

[Copilot]

Pull Request Overview

This PR adds a !mp addref <username> command that allows hosts to designate referees in multiplayer lobbies. The referee system enables designated users to execute commands on behalf of the host.

Key changes:

• Introduced MultiplayerRefereeTracker to manage referee state and automatically clear referees when leaving rooms
• Added command parsing logic to detect !mp addref commands and resolve usernames to API users
• Modified message handling to allow referees to execute chat commands on behalf of the host

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
MultiplayerMatchSubScreen.cs	Implements MultiplayerRefereeTracker component to manage referee list and handle room state changes
OsuGame.cs	Registers the new MultiplayerRefereeTracker component in the dependency injection container
OnlineMetadataClient.cs	Removes implementation from UpdateActivity method (appears unrelated to referee functionality)
StandAloneChatDisplay.cs	Adds referee command parsing, username resolution, and message handling logic for referee commands

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