Allow mapping annotations at class level

[dogawaf]

It would be cool to be able to define mapping at the class level.
This way, we could address the mapping for the _all field.

/**
 * @Flow\Entity
 * @ElasticSearch\Indexable("acme", typeName="organization")
 * @ElasticSearch\Mapping(index_name="_all", analyzer="standard_french")
 */
class Organization {

}

[dfeyer]

Nice idea, I don't have time to work on it, can you try to provide a basic PR @dogawaf I can help to make it mergeable