Add stickers converter

[Soheab]

Summary

This PR makes it possible to ask users to input a sticker by annotating an ext.commands.Command's parameter to any of the sticker types. This works similarly to the Attachment converter.

At the time of writing this description, I noticed that there is already a converter for guild stickers, but.. it only searches in cache for the name or id meanwhile this can get any uploaded stickers. But.. this implementation does conflict with the converter and I'm unsure what to do about that now.. (fml)

Another thing

It would be neat if you could specify a union of sticker types but I couldn't figure out a way to implement that cleanly since Attachment doesn't support that either.

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, ...)