This PR adds a new graphql_cache_is_object_cache_inabled filter that can allow 3rd party developers to dynamically enable/disable the object cache.

For example, (as mentioned in #177) a query param in the request, such as ?disable_cache could be read by this filter and disable the cache for that request.

This filter could be used like so:

add_filter( 'graphql_cache_is_object_cache_enabled', function( $enabled, $request ) {
  
  // disable caching when `?disable_cache` is included as a query param
  if (
        $_SERVER['REQUEST_METHOD'] === 'POST' &&
        isset( $_REQUEST['disable_cache'] )
    ) {
      return false;
    }

  return $enabled;
}, 10, 2 );