Allow per-parameter tab completion

Hello. I define the following command:
```java
@Subcommand("create")
public CompletableFuture handlePunishmentCreateCommand(CommandSender sender, Player target, PunishmentType type, int duration, String reason) { //... }
```

and I see that this framework automatically includes the tab completion for `PunishmentType`, which is an enum, but does not automatically suggest players for the `target` parameter.
Now I need to register tab completion just for this exact parameter to make it functional, but I could not find any way to do so.

It would be nice if that could be implemented.

Kind regards,
Lukas