Provide a schema for validation and improve config typing

I re-added the duplicate configuration of the id and label from the search entity because it would require refactoring the module; the module uses those values from the plugin configuration, not the search entity.

In analyzing the new schema against exported config data, I think there's still an issue with js_config.

Based on my experience refactoring the facets module, I think the problem will require reconsidering how that value is stored. Right now it doesn't fall nicely into sequence or mapping types.

sequence - can't be an associative array.
mapping - needs predictable keys specified in the schema.

Right now the code is storing that configuration as an associative array with variable keys which unfortunately doesn't fit into one of the above. It also looks like the full search page configuration is being stored a second time within this value? I also can't find where the configuration variable is actually used. Is that configuration still needed? We don't use this functionality, so ideally a site that does would take a look.

$js_config = [];
    foreach ($enabled as $plugin_id) {
      if ($this->jsPluginManager->hasDefinition($plugin_id)) {
        $instance = $this->jsPluginManager->createInstance($plugin_id, $this->configuration);
        $instance->submitConfigurationForm($form, $form_state);
        $js_config[$plugin_id] = $instance->getConfiguration();
      }
    }
    $this->configuration['js_config'] = $js_config;

I found https://www.drupal.org/project/config_inspector useful when looking at the results -