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/collab/src/tests/integration_tests.rs

@@ -9,7 +9,7 @@ use fs::{repository::GitFileStatus, FakeFs, Fs as _, RemoveOptions};
 use futures::StreamExt as _;
 use gpui::{
     px, size, AppContext, BackgroundExecutor, Model, Modifiers, MouseButton, MouseDownEvent,
-    TestAppContext,
+    SharedUrl, TestAppContext,
 };
 use language::{
     language_settings::{AllLanguageSettings, Formatter},
@@ -1828,7 +1828,7 @@ async fn test_active_call_events(
             owner: Arc::new(User {
                 id: client_a.user_id().unwrap(),
                 github_login: "user_a".to_string(),
-                avatar_uri: "avatar_a".into(),
+                avatar_uri: SharedUrl::network("avatar_a"),
             }),
             project_id: project_a_id,
             worktree_root_names: vec!["a".to_string()],
@@ -1846,7 +1846,7 @@ async fn test_active_call_events(
             owner: Arc::new(User {
                 id: client_b.user_id().unwrap(),
                 github_login: "user_b".to_string(),
-                avatar_uri: "avatar_b".into(),
+                avatar_uri: SharedUrl::network("avatar_b"),
             }),
             project_id: project_b_id,
             worktree_root_names: vec!["b".to_string()]