Matrix -> Discord Media Broken - Needs to be updated for MSC3916 Authenticated Media

[shady-ventures]

MSC3916 adds authentication to media requests. Specifically, the /_matrix/media/v3/download/ endpoint no longer works.

This line converts the media's MXC URL to the HTTP download URL.
https://github.com/matrix-org/matrix-appservice-discord/blob/develop/src/matrixeventprocessor.ts#L353

Unfortunately, matrix-bot-sdk does not support the new schema yet.
turt2live/matrix-bot-sdk#377

The bot should use the new media download endpoint somehow and add the Authorization header.