settings: use WpSpaJson to parse the settings

We need to use WpSpaJson to parse the values in WpSettings. This is because the
wireplumber configuration is written in JSON, so WpSettings should only hold
JSON values. To fix this, 2 API changes have been done:

- wp_settings_get_int() only accepts gint values, instead of gint64 values. This
is because the WpSpaJson API only parses int values, like spa_json_parse_int().

- wp_settings_get_string() now returns a newly allocated string, this is because
the string needs to be decoded in case it has quotes.

modules/module-settings.c

@@ -167,7 +167,7 @@ do_parse_settings (void *data, const char *location,
     }
     j = g_value_get_boxed (&item);
 
-    value = wp_spa_json_parse_string (j);
+    value = wp_spa_json_to_string (j);
     len = wp_spa_json_get_size (j);
     g_value_unset (&item);