Add VIP Search adapter

[nickdaugherty]

Adds a VIP Search adapter for es-wp-query.

Depends on #1 (will rebase or merge)

Testing

In isolation this doesn't do much and depends on Automattic/vip-go-mu-plugins#1593

1. Check out PR
2. Check out Add Search::query_es() helper vip-go-mu-plugins#1593 for mu plugins
3. In code, load the plugin and adapter:

wpcom_vip_load_plugin( 'es-wp-query' );
es_wp_query_load_adapter( 'vip-search' );

1. Then run an offloaded query, like this:

add_filter( 'pre_get_posts', function( $query ) {
	if ( ! $query->is_main_query() ) { 
		return $query;
	}

	if ( $query->is_category() || $query->is_home() ) {
		$query->set( 'es', true );
	}
} );

1. The above should register an ES query (in debug bar) and render the same posts that the DB query would

[brandon-m-skinner]

Seems reasonable enough