DDC-1899: [GH-385] set metadata for interface to be able to fetch entites by interface name

[doctrinebot]

Jira issue originally created by user @beberlei:

This issue is created automatically through a Github pull request on behalf of Burgov:

Url: #385

Message:

using the new ResolveTargetEntity functionality we noticed we needed another feature:

From the Symfony Bundle defining the interface, we'd like to be able to fetch entities by this very interface name, e.g.:

$em->find('Foo\BarBundle\Entity\PersonInterface', 1);

or

$em->getRepository('Foo\BarBundle\Entity\PersonInterface')->findAll();

This PR sets metadata for the interface when metadata for a class is loaded that the interface is configured for

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-385] was closed:
doctrine/dbal#385