EZP-22189 add optional filter criterion to LocationService::loadLocationChildren

https://jira.ez.no/browse/EZP-22189

Discussed with @lolautruche. We initially talked about a new language criterion but it seems to exist already (eZ\Publish\API\Repository\Values\Content\Query\Criterion\LanguageCode).

Here is the new parameter to allow filtering children loading.

I need help about 2 things I never lookup up:
* Integration tests
* Add parameter to REST requests