Convert auth_json from String to Path

The practical difference is that after this the "standard" path expansions
work (see
https://github.com/mopidy/mopidy/blob/3ecbc988d7965a391a43297c16b295b4190043f3/mopidy/config/types.py#L422
). These in turn allow sharing a config file between users that have
different names and home dirs.

mopidy_ytmusic/__init__.py

@@ -20,7 +20,7 @@ def get_default_config(self):
 
     def get_config_schema(self):
         schema = super().get_config_schema()
-        schema["auth_json"] = config.String(optional=True)
+        schema["auth_json"] = config.Path(optional=True)
         schema["auto_playlist_refresh"] = config.Integer(
             minimum=0, optional=True
         )