feat: add __len__ to ColorPalette

For downstream use being able to determine how many colors are in a
ColorPalette object can be useful. E.g., checking if a user has provided
a color palette that matches the number of labels. ColorPalette doesn't
currenly have a __len__ attribute meaning that `mypalette.colors` has
to be used for calculating lengths. This change adds __len__ to make
this more intutitive by allowing `len(mypalette)`.

supervision/draw/color.py

@@ -397,6 +397,15 @@ class ColorPalette:
         idx = idx % len(self.colors)
         return self.colors[idx]
 
+    def __len__(self) -> int:
+        """
+        Returns the number of colors in the palette.
+
+        Returns:
+            int: The number of colors.
+        """
+        return len(self.colors)
+
 
 def unify_to_bgr(color: Union[Tuple[int, int, int], Color]) -> Tuple[int, int, int]:
     """