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/remote_server/src/remote_editing_tests.rs

@@ -452,7 +452,7 @@ async fn test_remote_lsp(cx: &mut TestAppContext, server_cx: &mut TestAppContext
     });
 
     let mut fake_lsp = server_cx.update(|cx| {
-        headless.read(cx).languages.register_fake_language_server(
+        headless.read(cx).languages.register_fake_lsp_server(
             LanguageServerName("rust-analyzer".into()),
             lsp::ServerCapabilities {
                 completion_provider: Some(lsp::CompletionOptions::default()),
@@ -476,7 +476,7 @@ async fn test_remote_lsp(cx: &mut TestAppContext, server_cx: &mut TestAppContext
                 ..FakeLspAdapter::default()
             },
         );
-        headless.read(cx).languages.register_fake_language_server(
+        headless.read(cx).languages.register_fake_lsp_server(
             LanguageServerName("fake-analyzer".into()),
             lsp::ServerCapabilities {
                 completion_provider: Some(lsp::CompletionOptions::default()),
@@ -669,7 +669,7 @@ async fn test_remote_cancel_language_server_work(
     });
 
     let mut fake_lsp = server_cx.update(|cx| {
-        headless.read(cx).languages.register_fake_language_server(
+        headless.read(cx).languages.register_fake_lsp_server(
             LanguageServerName("rust-analyzer".into()),
             Default::default(),
             None,