gpui: Add opacity to support transparency of the entire element (#17132)

Release Notes:

- N/A

---

Add this for let GPUI element to support fade in-out animation.

## Platform test

- [x] macOS
- [x] blade `cargo run -p gpui --example opacity --features macos-blade`

## Usage

```rs
div()
    .opacity(0.5)
    .bg(gpui::black())
    .text_color(gpui::black())
    .child("Hello world")
```

This will apply the `opacity` it self and all children to use `opacity`
value to render colors.

## Example

```
cargo run -p gpui --example opacity
cargo run -p gpui --example opacity --features macos-blade
```

<img width="612" alt="image"
src="https://github.com/user-attachments/assets/f1da87ed-31f5-4b55-a023-39e8ee1ba349">

crates/gpui/src/color.rs

@@ -461,6 +461,16 @@ impl Hsla {
     pub fn fade_out(&mut self, factor: f32) {
         self.a *= 1.0 - factor.clamp(0., 1.);
     }
+
+    /// Returns a new HSLA color with the same hue, saturation, and lightness, but with a modified alpha value.
+    pub fn opacity(&self, factor: f32) -> Self {
+        Hsla {
+            h: self.h,
+            s: self.s,
+            l: self.l,
+            a: self.a * factor.clamp(0., 1.),
+        }
+    }
 }
 
 impl From<Rgba> for Hsla {