Add support for embed flags and update attachment flags

[DA-344]

Summary

• Adds EmbedFlags and EmbedMediaFlags.
• New flags parameter on Embed.__init__ and Embed.set_image.
• New flags type hint on _EmbedImageProxy protocol.
• New flags on AttachmentFlags

DDevs PR: discord/discord-api-docs#7353

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, ...)

[Rapptz]

Seems mostly okay.