Support filter-only searches

Problem/Motivation

Currently the module uses /core/modules/search/src/Controller/SearchController.php to render search pages.

The core controller doesn't call buildResults() unless the 'keys' parameter is present.

When filters are exposed, folks who are searching might want to only use the filters and not provide a keyword.

Proposed resolution

Lightweight approach to get around the requirement similar to how the node module search works to allow advanced searches without a keyword:

• Add a configuration option to allow for filter only searches
• Add an empty "keys" param to the query string. e.g. ?keys=
• Update docroot/modules/contrib/vertex_ai_search/src/Service/VertexSearchManager.php from line 260 to (or use the configuration option to determine whether to continue)
  

  if (!empty($searchKeys)) {
    // Set Keywords to be used in request query.
    $request->setQuery($searchKeys);
  } 

• Update docroot/modules/contrib/vertex_ai_search/src/Plugin/Search/VertexAISearch.php and add (with logic related to the new configuration setting)
  

  /**
   * {@inheritdoc}
   */
  public function isSearchExecutable() {
    // Allow search without keywords if configured.
    return TRUE;
  } 
  

• Adjust the no keywords message in docroot/modules/contrib/vertex_ai_search/vertex_ai_search.module if the search is configured to allow filter-only searches (likely want to check that a filter value exists)

Alternatively, this module could alter the route for the search controller and have its own logic.