feat: add coverart support

[mgoltzsche]

Implements the get_images method within the LibraryProvider so that it returns image URLs for tracks and albums. The returned image URLs point to a new Mopidy endpoint /subidy/coverart/{id} which forwards/reverse-proxies requests to the Subsonic server's getCoverArt endpoint. This is to authenticate with the Subsonic server, to avoid increasing configuration complexity and to support setups where the user cannot reach the Subsonic server directly.

Tested against https://github.com/BinaryBrain/Beetstream/pull/15

[mgoltzsche]

@Prior99 what's the state of this project? CI seems to be broken.