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/proto/src/proto.rs

@@ -406,6 +406,8 @@ messages!(
     (UpdateContext, Foreground),
     (SynchronizeContexts, Foreground),
     (SynchronizeContextsResponse, Foreground),
+    (LspExtSwitchSourceHeader, Background),
+    (LspExtSwitchSourceHeaderResponse, Background),
     (AddWorktree, Foreground),
     (AddWorktreeResponse, Foreground),
 );
@@ -528,6 +530,7 @@ request_messages!(
     (OpenContext, OpenContextResponse),
     (CreateContext, CreateContextResponse),
     (SynchronizeContexts, SynchronizeContextsResponse),
+    (LspExtSwitchSourceHeader, LspExtSwitchSourceHeaderResponse),
     (AddWorktree, AddWorktreeResponse),
 );
 
@@ -597,6 +600,7 @@ entity_messages!(
     CreateContext,
     UpdateContext,
     SynchronizeContexts,
+    LspExtSwitchSourceHeader
 );
 
 entity_messages!(