[Util] Add type hints to files in Util directory

[tambait]

Subject

This is a follow up of #6404 to add param & return type hints.

Changelog

### Added
- Added parameter and return type hints to files in the Util directory

[tambait]

Currently tests are failing because of #L78. I see in jordisala1991 PR #6366 that second parameter which is causing the issue is removed. Also, on #L72 of jordisala1991 PR AclInterface is replaced for MutableAclInterface etc. Can I assume it's ok to implement those and other changes from his PR into this PR?

[VincentLanglet]

IMHO:

public function getChildren()
    {
        return new self($this->current());
    }

Is correct

[VincentLanglet]

And the change of AclInterface to MutableAclInterface is ok.

[VincentLanglet]

I would assume that all his change are ok. I will still review your full PR anyway ;)

[VincentLanglet]

Thanks