(`usvg`) Option to preserve primitive shapes (rect, ellipse, polygon)

Nobody has claimed this yet.

Hi. First of all, I want to thank for such dedicated works.

I am using usvg as drop in replacement of svgo and svgson (in js ecosystem) for the usecase of..
1. optimizging part (✅)
2. resolved values (✅)
3. mapping them into my own struct (❌)

As usvg currently only have (by design) `Group`, `Path`, `Image`, `Text`
And the primitive shapes being all converted to Path, which is a solid, simple and clean choice. BUT for my case, I actually need to preserve such ``, ``, `` as non-path-resolved primitive node.

Wondering if this might be a consideration.
And the effort needed to have such as opt-in feature (willing to contribute)

## Suggestions

**A. Extend `Node::Path`**
make `Node::Path` have extra auxilary properties, passed down from original shapes

```rs
#[derive(Clone, Debug)]
pub struct Path {
pub(crate) id: String,
pub(crate) visible: bool,
pub(crate) fill: Option,
pub(crate) stroke: Option,
pub(crate) paint_order: PaintOrder,
pub(crate) rendering_mode: ShapeRendering,
pub(crate) data: Arc,
...
pub(crate) original_element_tag: SVGElementType
pub(crate) original_element_attributes: HashMap
}
```

**B. Introduce `Node::Rect` and so on**
Introduce `Node::Rect` as standard Node

```rs
#[derive(Clone, Debug)]
pub struct Rectangle {
pub(crate) id: String,
pub(crate) visible: bool,
pub(crate) fill: Option,
pub(crate) stroke: Option,
pub(crate) paint_order: PaintOrder,
pub(crate) rendering_mode: ShapeRendering,
...
}
```

**C. callback for collecting data while traversal**
Callback system or plugin-like system, where I can collect data if certain element is converted to path, I can store that, use the style output from `Node::Path` while knowing the original attributes, rx, etc..

Start with usvg's Node model, especially Node::Path and the conversion of primitive shapes into paths. Compare suggestions A, B, and C, then define an agreed opt-in design whose completion criteria specify how rect, ellipse, and polygon data are preserved while existing path behavior remains available.

Written by the indexing model from the issue text.