Accept ImageData when passing an image for an emoji in GuildId::create_emoji

[arqunis]

Previously, users were able to create the base64-encoded blob of data via CreateAttachment::to_base64, which returned a string. On next, this method had been replaced by encode, which returns an ImageData instead.

It is still the base64-encoded blob of data as before, but users were no longer able to access it directly. This wasn't too much of a problem, as many places that accepted a plain string switched to expecting ImageData. So for users, nothing practically has changed. Except GuildId::create_emoji, which still wanted a string, yet users couldn't reuse any ImageData they might have already had.

This rectifies the issue by not only switching image: &str to image: ImageData in GuildId::create_emoji, but also adding an accessor method for the blob of data in ImageData.