Add Rounded box gizmos

[lynn-lumen]

Objective

• Implement rounded cuboids and rectangles, suggestion of Tracking issue: Amazing Gizmos #9400

Solution

• Added Gizmos::rounded_cuboid, Gizmos::rounded_rect and Gizmos::rounded_rect_2d.
• All of these return builders that allow configuring of the corner/edge radius using .corner_radius(...) or .edge_radius(...) as well as the line segments of each arc using .arc_segments(...).

---

Changelog

• Added a new rounded_box module to bevy_gizmos containing all of the above methods and builders.
• Updated the examples 2d_gizmos and 3d_gizmos

Additional information

The 3d example now looks like this:

And this is the updated 2d example showcasing negative corner radius:

[JMS55]

Never reviewed gizmo stuff before, but lgtm.

[jgayfer]

Not a Gizmo export by any means, but ran the examples and overall looks good to me.

Could be nice to pull that default into a constant, but not blocking.

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1315 if you'd like to help out.