SW-9795 REST-API EAN

Search for EAN vai REST-API

http://jira.shopware.de/?ticket=SW-9795
`findOneBy` does not check for uniqueness, also the `ean` field is not unqiue in the database.

I put together a quick and dirty PR as an alternative, see: https://github.com/bcremer/shopware/pull/1/files.