Add generic slowmode channel manager to slowmode channel interface

[Sanduhr32]

Pull Request Etiquette

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

Changes

• Internal code
• Library interface (affecting end-user code)
• Documentation (maybe)
• Other: _____

Closes Issue: NaN

Description

Seems only logical to me to be able to use the generic manager, so you don't have to cast to 3 specific slow mode channels (Text, Thread or Forum).

Example use case from attempting to port JDA-Butler to JDA-v5

public static void announce(StandardGuildMessageChannel channel, Role role, MessageCreateData message, boolean slowmode)
{
    if (slowmode && channel instanceof ISlowmodeChannel)
    {
        ISlowmodeChannel slow = (ISlowmodeChannel) channel;
        ISlowmodeChannelManager<?, ?> slowMan = (ISlowmodeChannelManager<?, ?>) slow.getManager(); // currently resolves to ChannelManager and requires an unchecked cast
        slowMan.setSlowmode(30).queue(v -> slowMan.setSlowmode(0).queueAfter(2, TimeUnit.MINUTES));
    }

    role.getManager().setMentionable(true)                  // make role mentionable
        .flatMap(v -> channel.sendMessage(message))         // send announcement
        .flatMap(m -> {
            if (channel instanceof NewsChannel && channel.getGuild().getSelfMember().hasPermission(channel, Permission.MESSAGE_MANAGE))
                m.crosspost().queue();                      // publish if it's a news channel
            return role.getManager().setMentionable(false); // make role unmentionable
        })
        .queue();
    }

after overloading the manager return type the first if statement's body becomes

    if (slowmode && channel instanceof ISlowmodeChannel)
    {
        ISlowmodeChannel slow = (ISlowmodeChannel) channel;
        ISlowmodeChannelManager<?, ?> slowMan = slow.getManager();
        slowMan.setSlowmode(30).queue(v -> slowMan.setSlowmode(0).queueAfter(2, TimeUnit.MINUTES));
    }