Add full support for guild stickers

[jb3]

This extends the support we already had in Nostrum for guild stickers by adding all the missing API routes, structs and so on.

• Add all missing fields to sticker struct and move the enums to atoms
• Add a new cdn_url/1 method to the sticker struct for fetching the CDN URL
• Add all API routes required to fetch and manage guild stickers
• Add API route for fetching public sticker packs
• Update all caches to be able to handle sticker updates for dispatches to users
• Add tests for all of the above
• Change the permissions to reflect the new naming for manage_guild_expressions and the addition of the new create_guild_expressions permission

[jb3]

Thanks!