Add ability to customize center text in PieChart

* [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 Created a pie chart with center circle

## What did you expect to happen?

ℹ Have the ability to customize the font/color of the center circle text

## What happened instead?

ℹ Text is always 12 point font with labelOrBlack color

## Charts Environment

**Charts version/Branch/Commit Number: 4.1.0**
**Xcode version: 14.3 (14E222b)**
**Swift version: swift-driver version: 1.75.2 Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)**
**Platform(s) running Charts: iOS, ipadOS**
**macOS version running Xcode: 13.3.1 (a) (22E772610a)**

## Demo Project

ℹ [Demo](https://github.com/danielgindi/Charts/blob/master/ChartsDemo-iOS/Swift/Demos/PieChartViewController.swift)

Start with the PieChartViewController.swift demo linked in the issue, then trace how the pie chart's center text is configured and drawn. Done means callers can customize the center text's font and color instead of being limited to 12-point text in labelOrBlack.

Written by the indexing model from the issue text.