feat(REST): append additional information to the required User Agent

[QSmally]

Please describe the changes this PR makes and why it should be merged:

This PR allows you to append information about the bot (like a website, version) to the end of the (already required) user agent. As specified in the API documentations, users should be able to do so and are encouraged to do it.

Discord.js users could alternatively modify the constants file, where the user agent string is defined, but it can lead to them not following the API specifications.

After having a talk with a few people, I decided to implement a userAgentSuffix option to ClientOptions for people to do this. Types are already updated. An example of the usage is down below.

const { Client } = require("discord.js");
const botClient = new Client({
    userAgentSuffix: [`MyBot/${/* botversion */}`]
});

The implementation of this PR would append the array, joined by , , to the end of the already-default user agent.

• Default: DiscordBot ($djsurl, $djsversion) Node.js/$nodeversion
• With suffix: DiscordBot ($djsurl, $djsversion) Node.js/$nodeversion, MyBot/$botversion

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[DTrombett]

Why not making it a string instead of an array?

[QSmally]

Why not making it a string instead of an array?

It takes the approach of RFC7231. Each segment is divided, and it would be useful if those are reflected in the client option as well.

[DTrombett]

I think the validateOptions method should be updated as well

discord.js/src/client/Client.js

Line 468 in 72473e0

_validateOptions(options = this.options) {

[vladfrangu]

Optional destructuring, but otherwise LGTM

[kyranet]

I'm not a fan of this code, since it's the checks and user agent are done in every request rather than doing it only once (maybe in the RESTManager?).

I won't block, we can seek an alternative with a smaller performance impact later in another PR.

[QSmally]

@kyranet - I did not notice that the APIRequest was created on every request. Of course, now it's quite obvious and it was my mistake for not looking into it more. We can look into quite possibly creating the final user agent at startup of the bot, rather than on every request.