Add external touch handler for Pie Chart

[samir7osny]

Regarding the case of stacked pie charts on each other, the touch handler will work on one of them only, check this issue #1307 as well.

To try this case and understand the issue, you can go to this commit ceb4e18 and check sample 4.

This PR fixes this by adding the ability to calculate the touch response based on external parameter for the touched position, now you can use MouseRegion widget for example and add the stacked Pie charts inside it then pass the touched position to the charts and get the calculated response (check pie chart sample 4).

[imaNNeo]

There is a better solution to work with the low-level RenderBox/RenderObject and the HitTest topic.

fl_chart/lib/src/chart/base/base_chart/render_base_chart.dart

Line 109 in 803409c

bool hitTestSelf(Offset position) => true;

Basically, we need to run getResponseAtLocation() and return false if response is null (or something like that, you need to take a look)

fl_chart/lib/src/chart/base/base_chart/render_base_chart.dart

Lines 175 to 177 in 803409c

	/// Charts need to implement this class to tell us what [BaseTouchResponse] is available at provided [localPosition]
	/// When touch/pointer event happens, we send it to the user alongside the [FlTouchEvent] using [_touchCallback]
	R getResponseAtLocation(Offset localPosition);

btw, if you think it is a complicated topic (which is actually complicated), I can do that.