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);
    }

[staabm]

@ondrejmirtes could you rebase this branch so we can see how/whether mutation testing is doing here?

[ondrejmirtes]

Rebased

[staabm]

thanks.

test succeeded, as this PR does not contain any ->yes() or ->no() invocation, which means no mutations are generated. mutation testing jobs are green as expected.

[ondrejmirtes]

Sorry for this late review. Given this adds a new error, this should only be happening with Bleeding Edge.