Async suggestion support.

Please describe your suggestion Async support for overriding suggestion.

Please supply examples of the desired inputs and outputs

new PlayerArgument("player").overrideSuggestions(sender -> {
  return CompletableFuture.supplyAsync(() -> {
    final var players = new ArrayList<String>();
    players.add("db-player-1");
    players.add("db-player-2");
    players.add("db-player-3");
    return players.toArray(new String[0]);
  });
});

[JorelAli]

This is indeed possible! Massive kudos to the pull request from #249.

[JorelAli]

Implemented in 7.0.0.