Allow to use collection for block childrens

[VincentLanglet]

Subject

I am targeting this branch, because {reason}.

Closes #{put_issue_number_here}.

Changelog

### Added
- BlockInterface::addChild()
- BlockInterface::hasChild()

### Changed
- BlockInterface::getChildren() now returns iterable

### Deprecated
- BlockInterface::addChildren()
- BlockInterface::hasChildren()

[jordisala1991]

what is the advantage over iterable?

[VincentLanglet]

We're using count: https://github.com/sonata-project/SonataBlockBundle/blob/4.x/src/Model/BaseBlock.php#L224

[dmaicher]

so array<BlockInterface> would be fine then? 🤔

[VincentLanglet]

The goal is to allow PageBundle to use Collection, https://github.com/sonata-project/SonataPageBundle/blob/4.x/src/Entity/BaseBlock.php#L34

See sonata-project/SonataPageBundle#1442 or sonata-project/SonataPageBundle#1469 (comment)

[jordisala1991]

do we use addChild on page bundle?

[VincentLanglet]

No.

In the same way we have addChildren method in the PageInterface of PageBundle https://github.com/sonata-project/SonataPageBundle/blob/fed26c06856de8d6b1161d0f23001db7b8377e2b/src/Model/PageInterface.php#L189 which is not used. We could rename this to addChild.

If you had in mind to remove the addChild method. I'm not chocked having some add/set method in the interface/class which will only be used by the user.