Request `/v3` media endpoints over federation

[turt2live]

matrix-org/synapse#15196 but for MMR

See matrix-org/synapse#16701

[turt2live]

This is already done: https://github.com/turt2live/matrix-media-repo/blob/6aaa26fe7674062a98c27e8248cc07f0bee74417/pipelines/_steps/download/try_download.go#L57