e17_message_components - Custom Emoji for message components

[DrLarck]

Description

Hello, I think it would be a good thing to add an example of custom emoji usage for the e17_message_components example.

For instance:

use serenity::builder::CreateButton;
use serenity::model::id::EmojiId;
use serenity::model::application::component::ButtonStyle;

let custom_emoji = EmojiId(1234567890);  // the custom emoji
let button = CreateButton::default();
button
    .custom_id("custom_emoji_button_id")
    .style(ButtonStyle::Primary)
    .emoji(custom_emoji)
    .label("");

Enhancement

Could also add another enum for custom emojis, that would contain functions returning EmojiId.

Motivation

As a new Serenity user, I had a really bad time finding a way to use a custom emoji in a message component. I think that it could save a lot of time to newcomers.

Thank you.

EDIT: Adds Enhancement section