Authenticated media support

Matrix spec v1.11 was recently released, including MSC3916 - Authenticated Media. The plan is to have servers disable unauthenticated media access before August 2024, so we need to implement the new api soon to stay compatible.

The client API changes for MSC3916 have been merged into ruma, but the federation API changes have not (as of 2024-06-20). Conduit maintainers were talking about wanting to make a release for MSC3916 when the implementation was ready, but I don't know what state that's in currently. I didn't find any MRs.