clangd: Implement switch source/header extension (#14646)

Release Notes:

- Added switch source/header action for clangd language server (fixes
[#12801](https://github.com/zed-industries/zed/issues/12801)).

Note: I'm new to both rust and this codebase. I started my
implementation by copying how rust analyzer's "expand macro" LSP
extension is implemented. I don't yet understand some of the code I
copied (mostly the way to get the `server_to_query` in `clangd_ext.rs`
and the whole proto implementation).

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/editor/src/element.rs

@@ -165,6 +165,7 @@ impl EditorElement {
         });
 
         crate::rust_analyzer_ext::apply_related_actions(view, cx);
+        crate::clangd_ext::apply_related_actions(view, cx);
         register_action(view, cx, Editor::move_left);
         register_action(view, cx, Editor::move_right);
         register_action(view, cx, Editor::move_down);