Add support for redirecting media downloads (MSC3860)

[ser]

It would be a massive efficiency to deploy MSC3860 to this storage provider to allow storage to be served directly from S3 servers.

I would see it sending a s3 get_presigned_url() of a media file to the client to keep S3 server closed from public.

[clokep]

I agree that this would be a useful feature to support. 👍

[DMRobertson]

No disagreements from the Synapse team. We're not going to prioritise it, but would gladly review a PR.