Support for avatar decoration

[codeofandrin]

Summary

This PR adds support for avatar decoration.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[owocado]

I have observed on few users profiles, the avatar decoration asset URL is in the format of
https://cdn.discordapp.com/avatar-decorations/{user_id}/{avatar_decoration_hash}.png
probably for legacy or very early released avatar decos, havent dig too deep into it. Is it in the scope of this PR to address those cases?

[Rapptz]

I have observed on few users profiles, the avatar decoration asset URL is in the format of https://cdn.discordapp.com/avatar-decorations/{user_id}/{avatar_decoration_hash}.png probably for legacy or very early released avatar decos, havent dig too deep into it. Is it in the scope of this PR to address those cases?

This is the actual documented URL in the in-progress PR so it should be the one used.

[Rapptz]

If you have time please rebase this PR to the 2.4 release now that the docs PR is merged.

[codeofandrin]

Done

[Soheab]

Looks like the field is going to be changed or has already and returns an object now with the sku_id and asset
discord/discord-api-docs#6464

[owocado]

This change would be needed to prevent TypeError here.