Add `JDA#listenOnce`

[freya022]

Pull Request Etiquette

• [X] I have checked the PRs for upcoming features/bug fixes.
• [X] I have read the contributing guidelines.

Changes

• [X] Internal code
• [X] Library interface (affecting end-user code)
• [ ] Documentation
• [ ] Other: _____

Closes Issue: NaN

Description

Adds an helper method to listen for an event, once.

Example: Listening to a message from a channel and a user, after using a slash command:

final Duration timeout = Duration.ofSeconds(5);
event.reply("Reply in " + TimeFormat.RELATIVE.after(timeout) + " if you can!")
       .setEphemeral(true)
       .queue();

event.getJDA().listenOnce(MessageReceivedEvent.class)
        .filter(messageEvent -> messageEvent.getChannel().getIdLong() == event.getChannel().getIdLong())
        .filter(messageEvent -> messageEvent.getAuthor().getIdLong() == event.getUser().getIdLong())
        .timeout(timeout, () -> {
            event.getHook().editOriginal("Timeout!").queue();
        })
        .submit()
        .onSuccess(messageEvent -> {
            event.getHook().editOriginal("You sent: " + messageEvent.getMessage().getContentRaw()).queue();
        });