feat: Support scrobbling played songs to ListenBrainz

[chme]

This PR adds support for submitting "listens" (played songs) to ListenBrainz (https://listenbrainz.org).

ListenBrainz offers similar functionality as Last.fm, but the data is open (see their About page for their stated goals).

Integration of ListenBrainz in OwnTone would allow users to use ListenBrainz to keep a (public) listening history instead of using the proprietary Last.fm service.

This PR implements:

• Support submitting listens (scrobble) to ListenBrainz.
• Add JSON API endpoints to manage ListenBrainz auth token.

Note, that a minor refactoring was done in cc0d2ff.

I am opening this as draft, to see if there is interest in adding this functionality to OwnTone. And if yes, if the approach is OK.

Open topics to finalize the integration are:

• Document new JSON API endpoints
• Add ListenBrainz settings page in web interface

[ejurgensen]

Never heard about listenbrainz, but I think it we be fine to include. I added a few minor comments, but you are welcome to merge it when you think it's ready.

[ejurgensen]

Looks good. Small note: The if(x) for keyval_clear(x) is redundant, keyval_clear is no-op if the pointer is null.

[chme]

Thanks for taking the time to look into this!
I did the last change you recommended (makes the code a bit more readable) and rebased on master. I will test it over the weekend and if I find no issues, will merge it.