Add `IndexIf` annotation to conditionally index an entity

[keichinger]

In real world applications it's not uncommon to conditionally index entities. To automate the entire process a good solution to this scenario would be to introduce a new @Search\IndexIf method annotation. When instance of Foo is about to being indexed, we're checking for the existence of the @Search\IndexIf annotation. When the underlying method returns true the entity is indexed, otherwise it's skipped.

Like this:

use Becklyn\SearchBundle\Mapping as Search;
// ...

/**
 * @Search\Item()
 */
class Foo implements SearchableEntityInterface
{
    // ...

    /**
     * @Search\IndexIf()
     */
    public function getWhetherThisEntityShouldBeIndexedOrNot () : bool
    {
        return /* condition(s) */;
    }
}