Authenticated media and animated thumbnails support

[KitsuneRal]

This commit implements a proxy layer (almost entirely in terms of existing facilities - Connection::downloadFile()/DownloadFileJob and Connection::getThumbnail()/MediaThumbnailJob respectively) to spare clients from having to figure out which API, old or new, they should use to retrieve media on any given homeserver. In all honesty, the code for the new API hasn't been tested yet but it's almost entirely based on the generated API classes, so nothing should go wrong because our CS API descriptions are perfect, right? :-D

Implements MSC3916 and, while at it, [MSC2705](https://github.com/matrix-org/matrix-spec-proposals/pull/2705] (animated thumbnails).

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
2 Accepted issues

Measures
0 Security Hotspots
22.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud