Changes

Add support for stickers (#1474)

* add support for stickers

* idk why it reformatted

* fix docs

* jeez

* add MessageSticker#getTags

* typo

* add missing copyright & class level docs

* create a constant for sticker url

* add missing UnsupportedOperationExceptions to docs

* clarify what asset getters are

if anyone has a better explanation of what the asset hash is, let me know, because i just copied this from other docs

* publicize MessageSticker#STICKER_URL and add docs

* remove copyright end year

* apply some suggestions from the review

* rename the asset methods + improve the docs + add extensions to the formats

* rename getStickerUrl to getAssetUrl

* Update src/main/java/net/dv8tion/jda/api/entities/MessageSticker.java

Co-authored-by: Austin Keener <keeneraustin@yahoo.com>

* Apply suggestions from code review

Co-authored-by: Florian Spieß <business@minnced.club>

* Apply suggestions from code review

Co-authored-by: Florian Spieß <business@minnced.club>

* comment out preview asset hash method and add checks

* better wording

* remove commented out method

Co-authored-by: Austin Keener <keeneraustin@yahoo.com>
Co-authored-by: Florian Spieß <business@minnced.club>
 (commit: 2b998d2)