Add shadow to line chart's tooltip

[Sosssen]

hi:)

this PR resolves #1767

i've added 3 params to LineTouchTooltipData for tooltip shadow:

• shadowColor - color of the shadow
• shadowBlur - sigma value of mask filter blur (how much will shadow be blurred)
• shadowOffset - offset of the shadow (related to tooltip)

please check the image below to see example of changes:)

and the code code of LineTouchTooltipData from example image:

LineTouchData get lineTouchData => LineTouchData(
        touchTooltipData: LineTouchTooltipData(
          getTooltipColor: (touchedSpot) => Colors.black,
          shadowColor: Colors.white.withOpacity(0.25),
          shadowBlur: 4,
          shadowOffset: const Offset(0, 4),
        ),
      );

[seniorb]

Would be great if this could be added to the barchart tooltip as well!