PieChart backdrop shadow

**Describe the solution you'd like**

I'd love to create a pie chart with a backdrop shadow like this:

I search for existing issues and through the documentation and did not find any solution to this issue. Is this feature possible to add?

**Describe alternatives you've considered**
I tried using flutter alternatives but did not find any.

Start at the PieChart entry point and review the existing pie-chart documentation and rendering options. Compare the requested example with current behavior, then define a configurable backdrop-shadow behavior and verify that the resulting chart matches the request.

Written by the indexing model from the issue text.