Add option to allow customization of the thickness of the slider bar

## Describe the Feature
It would be great to allow the option to customize the thickness of the slider bar itself. Something like a `thickness` prop.

Start by locating the React Native Slider prop definitions and the platform implementations behind the iOS Slider and Android SeekBar. Check how existing props cross the platform boundary and determine the expected behavior for a new thickness prop. Done means the slider bar thickness can be customized on both supported platforms.

Rédigé par le modèle d'indexation à partir du texte de l'issue.