Add SweepGradient (#17368)

This PR adds a SweepGradient class, extending Gradient to expose the engine's ui.Gradient.sweep shader.

Similar to LinearGradient and RadialGradient - SweepGradients can be used in a BoxDecoration or passed to a Paint's shader.
diff --git a/AUTHORS b/AUTHORS
index 4516816..ab63cf6 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -23,3 +23,4 @@
 Tetsuhiro Ueda <najeira@gmail.com>
 Dan Field <dfield@gmail.com>
 Noah Groß <gross@ngsger.de>
+Victor Choueiri <victor@ctrlanddev.com>