[db,library] Add "scan_kind" field to playlists, directories and files

(db upgrade to v22.00)

`scan_kind` identifies the library "scanner" component that created the
item and is responsible to keep it up to date (rescan).

The library update now supports passing a `scan_kind` to update only the
items of one particular "scanner". This allows e. g. to only update the
item from the Spotify library or only update the RSS feeds.

The OwnTone database is upgraded to v22.00 and the `scan_kind` columns
in `files`, `playlists`, `directories` are identified by:

 1. Check if item is part of a RSS playlist (podcast RSS feed), they
belong to the "rssscanner"
 2. Check if item has a Spotify `virtual_path`, they belong to the
"spotifyscanner"
 3. Remaining items belong to the "filescanner"

src/mpd.c

@@ -3203,7 +3203,7 @@ mpd_command_update(struct evbuffer *evbuf, int argc, char **argv, char **errmsg,
       return ACK_ERROR_ARG;
     }
 
-  library_rescan();
+  library_rescan(0);
 
   evbuffer_add(evbuf, "updating_db: 1\n", 15);