drag and drop images from web into copilot chat

[justschen]

currently, attaching images supports a few differnt attachment types, including drag and drop. we want to support drag and drop directly from web.

ways we can do this:

1. drag and drop the image URL and send the URL to the model (cc. @benibenj for URL attachments)
2. drag and drop the image and download/convert to base64.

[sbatten]

Re-opening based on last minute changes to the release