Smooth out segment transitions

**Is your feature request related to a problem? Please describe.**
I would like to smooth the transition of my segments. When I add a Layer to mapbox I can pass the option : ```layout':{
"line-join": "round",
"line-cap": "round"
}``` and my transitions are great.

**Describe the solution you'd like**
Having access to a function that can append options like [layout or paint](https://docs.mapbox.com/help/glossary/layout-paint-property/) to the layer.