Check field names of orderBy parameter in findBy and findOneBy

[whataboutpereira]

Check valid field names are used in orderBy parameter of findBy and findOneBy repository methods (issue #674).

Unfortunately this won't catch a nested call like this:

    public function findAll(array $orderBy = ['name' => 'ASC']): array
    {
        return $this->findBy([], $orderBy);
    }