Add experimental LSP-based context retrieval system for edit prediction (#44036)

To do

* [x] Default to no context retrieval. Allow opting in to LSP-based
retrieval via a setting (for users in `zeta2` feature flag)
* [x] Feed this context to models when enabled
* [x] Make the zeta2 context view work well with LSP retrieval
* [x] Add a UI for the setting (for feature-flagged users)
* [x] Ensure Zeta CLI `context` command is usable

---

* [ ] Filter out LSP definitions that are too large / entire files (e.g.
modules)
* [ ] Introduce timeouts
* [ ] Test with other LSPs
* [ ] Figure out hangs

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/settings/src/settings_content/language.rs

@@ -62,6 +62,8 @@ impl merge_from::MergeFrom for AllLanguageSettingsContent {
 pub struct FeaturesContent {
     /// Determines which edit prediction provider to use.
     pub edit_prediction_provider: Option<EditPredictionProvider>,
+    /// Enables the experimental edit prediction context retrieval system.
+    pub experimental_edit_prediction_context_retrieval: Option<bool>,
 }
 
 /// The provider that supplies edit predictions.