Adds support for user's avatar decorations

[ItsTheSky]

Pull Request Etiquette

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

• [X] Internal code
• [X] Library interface (affecting end-user code)
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Closes Issue: NaN

Description

Adds support for the user's decoration avatar. Discord docs are outdated about the user's object, check this PR for the "updated" endpoints.

It's my first PR there, hope I did not break anything 😓

[freya022]

avatar_decoration_data is also available on Members, please add Member#getAvatarDecoration as well as Member#getEffectiveAvatarDecoration

[ItsTheSky]

Do I use the same object present in User for adding support in Member?

[freya022]

Yes, same object, but stored on the member, just like any member profile element that's also present on an user

[freya022]

As Member and User have a decoration, might as well make AvatarDecoration be its own class, outside of User

[DV8FromTheWorld]

@ItsTheSky what is the status here?