Add disconnect API call

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -53,4 +53,6 @@ public interface Player extends CommandSource, InboundConnection, ChannelMessage
      * @return a new connection request
      */
     ConnectionRequestBuilder createConnectionRequest(@NonNull ServerInfo info);
+
+    void disconnect(Component reason);
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -134,6 +134,11 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         return new ConnectionRequestBuilderImpl(info);
     }
 
+    @Override
+    public void disconnect(Component reason) {
+        connection.closeWith(Disconnect.create(reason));
+    }
+
     public VelocityServerConnection getConnectedServer() {
         return connectedServer;
     }