Add support for PieChart colorAxis options

This patch adds support for the colorAxis options for the Pie Charts chart type.

With this patch the colorAxis option can be used to set a custom range of colours for the Pie chart display. For example:

						const contents = new Drupal.Charts.Contents();
						if (drupalSettings.hasOwnProperty('charts') && drupalSettings.charts.highcharts.global_options) {
						  Highcharts.setOptions(drupalSettings.charts.highcharts.global_options);
						}
						once('charts-highchart', '.charts-highchart', context).forEach(function (element) {
						  const id = element.id;
						  let config = contents.getData(id);
						  config.legend.enabled = false;
						  delete config.colors;
						  config.colorAxis = {};
						  config.colorAxis.minColor = "#e3cccf";
						  config.colorAxis.maxColor = "#772310";
						  if (!config) {
						    return;
						  }

						  config.chart.renderTo = id;
						  new Highcharts.Chart(config);
						  if (element.nextElementSibling && element.nextElementSibling.hasAttribute('data-charts-debug-container')) {
						    element.nextElementSibling.querySelector('code').innerText = JSON.stringify(config, null, ' ');
						  }
						}); 

Which result will be like this(Color is different):