spotify: librespot debug logging

In order to better diagnose issues with the spotify plugin (and with librespot itself), can we make it easier for end-users to provide librespot debug logs? I've added this facility to my fork by routing librespot's logs to GStreamer's logging mechanism under a new "librespot" gst::DebugCategoryLogger.

Would a PR with this be welcome? Or is there an alternate preferred mechanism? kingosticks/gst-plugins-rs@82f81031