Adding corner radius on pie charts values

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?
I have created a pie chart using a chart
[Issue is Here](https://stackoverflow.com/questions/61257205/ios-custom-pie-charts-drawing)

## What did you expect to happen?
I am getting expected result but I wanted some customization on this Pie Chart

## What happened instead?
It is ok but I need some change

## Charts Environment

**Charts version/Branch/Commit Number:**
**Xcode version:** 11.4
**Swift version:** 5
**Platform(s) running Charts:** iOS 11
**macOS version running Xcode:** Catelina

## Demo Project