Introduce a new sendMultimediaMessage in lib/actions/message-actions that hits the create_multimedia_message endpoint with the updated payload (mediaMessageContents instead of mediaIDs).
This endpoint is consumed by input-state-container on native, but not on web. We continue to use the legacySendMultimediaMessage on web to avoid breaking things.
(As of this diff, the keyserver has everything it needs in multimediaMessageCreationResponder(...) to properly associate uploads with media with messages)
Depends on D5062