Add custom extra dynamic data for FlSpot

Add custom extra dynamic data for FlSpot, then we can easily display the custom information of each data point in LineTouchTooltipData.

For example:
FlSpot(double x, double y, dynamic data);

touchTooltipData: LineTouchTooltipData(
fitInsideHorizontally: true,
fitInsideVertically: true,
getTooltipItems: (List touchedBarSpots) {
return touchedBarSpots.map((touchedSpot) {
dynamic data = touchedSpot.data;
}
}
);

Start by locating FlSpot and LineTouchTooltipData, then trace how a spot becomes a LineBarSpot for tooltip callbacks. Add support for carrying custom per-point data through that path and verify that getTooltipItems can access it as shown in the issue example.

Written by the indexing model from the issue text.