add clustering/related searches support

Adds tests and clustering support to search service.

Block to come tomorrow.

I will work on block with @pasan.gamage tomorrow

1. +++ b/src/Plugin/Block/RelatedSearchesBlock.php
   @@ -0,0 +1,34 @@
   +
   

   nit: superfluous empty line here

2. +++ b/src/Plugin/Block/RelatedSearchesBlock.php
   @@ -0,0 +1,34 @@
   +    $searcher = \Drupal::service('google_appliance.search');
   +    $routeMatch = \Drupal::service('current_route_match');
   

   nit: these need to be injected by making the block implement ContainerFactoryPluginInterface, see the existing SearchBlock for example of how to inject services.

3. +++ b/src/Plugin/Block/RelatedSearchesBlock.php
   @@ -0,0 +1,34 @@
   +  public function getCacheContexts() {
   

   nit: needs a docblock - {@inheritdoc} will suffice

And we need to make the test pass