Gadgets can change the search API

```
mw.config.set('wgVectorSearchClient', {

   fetchByTitle: function( query, domain, limit ) {
      var xhr = fetch('http://' + domain + '/w/rest.php/v1/search/title?q=banana')
      .then(function (resp) {
          return resp.json();
      }).then(function (json) {
         return {
             results: json.pages
         }
      });
      return {
         fetch: xhr,
         abort: function() {}
      }
   }
})
```

This should be the absolute minimum to allow API clients to configure
the search. This should be considered an interim solution to buy us time to work out a more
elegant way to do this e.g. do this in the API itself…

Bug: T262566
Change-Id: Iac6f2551bed911980064dcb023193f800df0934f

resources/skins.vector.search/App.vue

@@ -3,6 +3,7 @@
 		<wvui-typeahead-search
 			id="searchform"
 			ref="searchForm"
+			:client="getClient"
 			:domain="domain"
 			:footer-search-text="$i18n('searchsuggest-containing').escaped()"
 			:suggestions-label="$i18n('searchresults').escaped()"
@@ -56,6 +57,14 @@ module.exports = {
 		}
 	},
 	computed: {
+		/**
+		 * Allow wikis eg. Hebrew Wikipedia to replace the default search API client
+		 *
+		 * @return {void|Object}
+		 */
+		getClient: function () {
+			return mw.config.get( 'wgVectorSearchClient', undefined );
+		},
 		language: function () {
 			return mw.config.get( 'wgUserLanguage' );
 		},