Add ListenBrainz support
ListenBrainz is a new open source and open data alternative to Last.fm, allowing users to submit their "listens" to the service. ListenBrainz provides its own API, but there is also a Audioscrobbler / Last.fm compatibility API
As GNOME Music already supports Last.fm scrobbling I think it would be fitting to also support an open and modern alternative. I am not entirely sure how such support can fit nicely into GNOME music with its simplified user interface, which currently avoids any configuration. ListenBrainz support requires at least the configuration of a user token, which the user can get from the ListenBrainz profile page.