Add spec and cutover support for attachments key on interaction response objects

[IGJoshua]

As noted in the documentation here: https://discord.com/developers/docs/interactions/receiving-and-responding#interaction-response-object-messages an attachments key should be noted in the specs for interaction responses.

There is currently a bug in Discord where the attachments key is read off the root object instead of the data key. There may be current users of discljord who are writing against this key. It should be considered to make a mitigation in a hotfix version to take the attachments key off the root of the interaction response object and merge it into the data key before the request is sent.

[IGJoshua]

So I looked at this again and I also need to just add support for attachments, because currently that's not done with the multipart stuff that we currently have set up.