feat(User): support user banners

[Koyamie]

Please describe the changes this PR makes and why it should be merged:
With the recent releases of user banners for everyone, Discord confirmed they will add the banner image hash to the user object.
See discord/discord-api-docs#3095 (comment)

This PR supports the banner field.

Do not merge until the field is accessible via the API.

Status and versioning classification:

• This PR changes the library's interface (methods or parameters added)

[DTrombett]

Typings for User class need to be updated.

[Koyamie]

I am not familiar with typings but this should do the job? Tell me if I missed something

[DTrombett]

They should be ok 👍

[GoldenAngel2]

User.equals needs to be updated

👍

[u9g]

In what situation would the users be equal but the banners be unequal that makes the equal function fail?

[DTrombett]

That's the same as other properties. I guess it's there to check two users object that may refer to the same user.

[advaith1]

banner and banner_color are documented in discord/discord-api-docs#3448 and are currently accessible via the canary API (in ClientOptions: http: {headers: {'x-debug-options': 'canary'}} or http: {api: 'https://canary.discord.com/api'})

[Koyamie]

I'll need someone else to take care of that PR because I won't be available for the next 2 weeks and Discord seems to be slowly releasing the banner on the API.

[iCrawl]

@advaith1 will be taking over in a new PR.