gpui: Improve img element to support load from Assets (#15482)

Release Notes:

- N/A


Currently, the `img` element provided by GPUI only supports FilePath or
URL, but in actual applications we need to let `img` load an image
embedded in Assets.

The `svg` element can currently support this, but `img` cannot.

For example:

We have such an Assets directory:

```
assets
|- icons
|- images
|--- foo.png
```

```rs
// If give a path, considered an Asset
img("images/foo.png");
// If give a URI, considered a Remote image
img("https://foo.bar/images/foo.png");
// If give a PathBuf, considered a Local file
img(PathBuf::from("path/to/foo.png"));
```


## Example test

```
cargo run -p gpui --example image 
```

<img width="827" alt="image"
src="https://github.com/user-attachments/assets/e45dcf7f-4626-4fb0-aca9-9b6e1045a952">

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/gpui/src/asset_cache.rs

@@ -1,4 +1,4 @@
-use crate::{SharedUri, WindowContext};
+use crate::{SharedString, SharedUri, WindowContext};
 use collections::FxHashMap;
 use futures::Future;
 use parking_lot::Mutex;
@@ -11,6 +11,7 @@ use std::{any::Any, path::PathBuf};
 pub(crate) enum UriOrPath {
     Uri(SharedUri),
     Path(Arc<PathBuf>),
+    Asset(SharedString),
 }
 
 impl From<SharedUri> for UriOrPath {