Message attachments (multiple photos, files, other messages for forwarding, etc)

Many messengers allow attaching one or multiple media (file, photo, video), when user composing message text, and even other messages (eg for forwarding message from one room to other). But Matrix protocol now only can send one media per message (without text), so if user want to send several photos and text, it will generate several separate messages.

I can't find feature request about adding attachments to message, so create this new feature request.

As implementation, this can be simple field `m.attachments`, with list of `msc://` urls for media, or `event_id`'s for attach messages, eg for forwarding message to other room, in event `content`. Rendering attachments can be client-side feature.

No file or test is named. Start by reviewing the Matrix event content specification and the 19-comment discussion, then trace how media and forwarded messages are currently represented. Done means the protocol design clearly defines attachment data, its relationship to message text, and the boundary between protocol data and client-side rendering.

Written by the indexing model from the issue text.