As of today the only available option is to connect device with mini-jack either to a Play:5 or thru a Connect as far as i've figured out.
Best answer by Airgetlam
First, it would be up to Spotify to do such integration, so you should be petitioning them. Sonos would only be involved inasmuch as QA and approving it.
Sonos, for better or worse, generally doesn't share timelines with us. While they do make marketing announcements for large features. Apple Music, Amazon Voice control and Sonos control in the Spotify app being the standouts. For any other features, we just don't know until they release.