Proposal for a media information API

[turt2live]

Documentation: https://docs.google.com/document/d/1yNnOIFuYYu33YVVhOMPZoNQ8YBGcSfSiO1bTe1NwPHI/edit?usp=sharing
Resolves: matrix-org/matrix-spec#196

[anoadragon453]

I was working on a server-side feature that this would've made much easier. Mind if I markdownify this and turn it into an MSC?

[turt2live]

Oh please do. Note that it's partially implemented by matrix-media-repo already as an unstable endpoint.

[anoadragon453]

This doc has been turned into an MSC here: #2380