gpui: Support loading image from filesystem (#6978)

This PR implements support for loading and displaying images from a
local file using gpui's `img` element.

API Changes:
- Changed `SharedUrl` to `SharedUrl::File`, `SharedUrl::Network`

Usage:
```rust
// load from network
img(SharedUrl::network(...)) // previously img(SharedUrl(...)

// load from filesystem
img(SharedUrl::file(...))
```

This will be useful when implementing markdown image support, because we
need to be able to render images from the filesystem (relative/absolute
path), e.g. when implementing markdown preview #5064.

I also added an example `image` to the gpui crate, let me know if this
is useful. Showcase:
<img width="872" alt="image"
src="https://github.com/zed-industries/zed/assets/53836821/b4310a26-db81-44fa-9a7b-61e7d0ad4349">

**Note**: The example is fetching images from [Lorem
Picsum](https://picsum.photos) ([Github
Repo](https://github.com/DMarby/picsum-photos)), which is a free
resource for fetching images in a specific size. Please let me know if
you're okay with using this in the example.

crates/client/src/user.rs

@@ -707,7 +707,7 @@ impl User {
         Arc::new(User {
             id: message.id,
             github_login: message.github_login,
-            avatar_uri: message.avatar_url.into(),
+            avatar_uri: SharedUrl::network(message.avatar_url),
         })
     }
 }