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_ui/src/chat_panel/message_editor.rs

@@ -365,7 +365,7 @@ impl Render for MessageEditor {
 mod tests {
     use super::*;
     use client::{Client, User, UserStore};
-    use gpui::TestAppContext;
+    use gpui::{SharedUrl, TestAppContext};
     use language::{Language, LanguageConfig};
     use rpc::proto;
     use settings::SettingsStore;
@@ -392,7 +392,7 @@ mod tests {
                         user: Arc::new(User {
                             github_login: "a-b".into(),
                             id: 101,
-                            avatar_uri: "avatar_a-b".into(),
+                            avatar_uri: SharedUrl::network("avatar_a-b"),
                         }),
                         kind: proto::channel_member::Kind::Member,
                         role: proto::ChannelRole::Member,
@@ -401,7 +401,7 @@ mod tests {
                         user: Arc::new(User {
                             github_login: "C_D".into(),
                             id: 102,
-                            avatar_uri: "avatar_C_D".into(),
+                            avatar_uri: SharedUrl::network("avatar_C_D"),
                         }),
                         kind: proto::channel_member::Kind::Member,
                         role: proto::ChannelRole::Member,