DropZone: Allow overriding children

[swissspidy]

Is your feature request related to a problem? Please describe.

I am trying to use the DropZone component together with Draggable to implement a simple sorting mechanism.

While doing so, I realized I can change the default "Drop files to upload" label, but not the upload icon.

In the 'BlockDropZone' component, this is "solved" by just hiding the content using CSS, which isn't really nice, but currently the only way to achieve this.

Describe the solution you'd like

It would be way easier if we could pass custom content as children or have some other way to remove the default content in DropZone.

[youknowriad]

Just nothing that there's a __unstableUseDropZone react hook allowing alternative UIs for drop zones. We should probably consider making this API stable instead of adding children support to DropZone.

Ideally, we'd also avoid the need for a DropzoneProvider at some point.