assistant: Overhaul provider infrastructure (#14929)

<img width="624" alt="image"
src="https://github.com/user-attachments/assets/f492b0bd-14c3-49e2-b2ff-dc78e52b0815">

- [x] Correctly set custom model token count
- [x] How to count tokens for Gemini models?
- [x] Feature flag zed.dev provider
- [x] Figure out how to configure custom models
- [ ] Update docs

Release Notes:

- Added support for quickly switching between multiple language model
providers in the assistant panel

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/settings/src/settings.rs

@@ -21,7 +21,7 @@ pub use settings_store::{
 pub struct SettingsAssets;
 
 pub fn init(cx: &mut AppContext) {
-    let mut settings = SettingsStore::default();
+    let mut settings = SettingsStore::new(cx);
     settings
         .set_default_settings(&default_settings(), cx)
         .unwrap();