Add shadow to line chart's tooltip

hi, thanks for very nice package:)

**Is your feature request related to a problem? Please describe**
i've just started using fl_chart for line charts in my project and i'm missing an option to add shadow to tooltip:) please check the attached image below

**Describe the solution you'd like**
i feel like the best solution would be to add some shadow parameters to `LineTouchTooltipData`

**Describe alternatives you've considered**
i've tried working-around this effect by playing with available `LineTouchTooltipData` params but nothing was close to desired effect:)

**Additional context**

Start by locating LineTouchTooltipData and the line-chart tooltip rendering that consumes it. Add configurable shadow parameters for the tooltip and verify that the rendered tooltip can produce the requested shadow effect.

Written by the indexing model from the issue text.