gradient designing is not available for the slices in the fortune wheel

[Tarunbhati100]

while using the fortune wheel package I found that the gradient design is not available in the fortune wheel package which is a very good alternative to design the cool wheel designs.

I have added the gradient design functionality which enables the user to create the gradient designs for each fortune item to use in one of my project using the flutter_fortune_wheel package source code

I think it should also be included in the package so if you would like I can create a PR for this.

Sharing some design I have built using the gradient functionality added.