Add custom icon for Anthropic hosted models (#16436)

This commit adds a custom icon for Anthropic hosted models.


![CleanShot 2024-08-18 at 15 40
38@2x](https://github.com/user-attachments/assets/d467ccab-9628-4258-89fc-782e0d4a48d4)
![CleanShot 2024-08-18 at 15 40
34@2x](https://github.com/user-attachments/assets/7efaff9c-6a58-47ba-87ea-e0fe0586fedc)


- Adding a new SVG icon for Anthropic hosted models.
  - The new icon is located at: `assets/icons/ai_anthropic_hosted.svg`
- Updating the LanguageModel trait to include an optional icon method
- Implementing the icon method for CloudModel to return the custom icon
for Anthropic hosted models
- Updating the UI components to use the model-specific icon when
available
- Adding a new IconName variant for the Anthropic hosted icon

We should change the non-hosted icon in some small way to distinguish it
from the hosted version. I duplicated the path for now so we can
hopefully add it for the next release.

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -4135,7 +4135,7 @@ impl Render for ContextEditorToolbarItem {
                                             (Some(provider), Some(model)) => h_flex()
                                                 .gap_1()
                                                 .child(
-                                                    Icon::new(provider.icon())
+                                                    Icon::new(model.icon().unwrap_or_else(|| provider.icon()))
                                                         .color(Color::Muted)
                                                         .size(IconSize::XSmall),
                                                 )