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/open_ai/src/open_ai.rs

@@ -77,14 +77,14 @@ impl Model {
         }
     }
 
-    pub fn id(&self) -> &'static str {
+    pub fn id(&self) -> &str {
         match self {
             Self::ThreePointFiveTurbo => "gpt-3.5-turbo",
             Self::Four => "gpt-4",
             Self::FourTurbo => "gpt-4-turbo-preview",
             Self::FourOmni => "gpt-4o",
             Self::FourOmniMini => "gpt-4o-mini",
-            Self::Custom { .. } => "custom",
+            Self::Custom { name, .. } => name,
         }
     }