Improve server list ping, especially for legacy MC versions.

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

@@ -32,6 +32,12 @@ public interface ProxyServer {
      */
     Collection<Player> getAllPlayers();
 
+    /**
+     * Returns the number of players currently connected to this proxy.
+     * @return the players on this proxy
+     */
+    int getPlayerCount();
+
     /**
      * Retrieves a registered {@link ServerInfo} instance by its name.
      * @param name the name of the server