PieChart: Support combining smaller slices into an other or rest category

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: Pie charts with many different series can be difficult to read, and limiting the number of rows displayed in a pie chart results in incorrect results (because only the displayed rows are taken into account for the percentage, not all the rest). **What would you like to be added**: Ability to combine values below X% into a single "Others" slice. **Who is this feature for?** Anyone that tries to display a pie chart with many series. See https://github.com/grafana/grafana/discussions/38307 and https://github.com/grafana/grafana/issues/35960, the previous iterations of this same request.