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/proto/zed.proto

@@ -131,7 +131,7 @@ message Envelope {
         UpdateUserPlan update_user_plan = 234;
         UpdateDiffBase update_diff_base = 104;
         AcceptTermsOfService accept_terms_of_service = 239;
-        AcceptTermsOfServiceResponse accept_terms_of_service_response = 240; // current max
+        AcceptTermsOfServiceResponse accept_terms_of_service_response = 240;
 
         OnTypeFormatting on_type_formatting = 105;
         OnTypeFormattingResponse on_type_formatting_response = 106;
@@ -264,15 +264,18 @@ message Envelope {
 
         GetSignatureHelp get_signature_help = 217;
         GetSignatureHelpResponse get_signature_help_response = 218;
+
         ListRemoteDirectory list_remote_directory = 219;
         ListRemoteDirectoryResponse list_remote_directory_response = 220;
         UpdateDevServerProject update_dev_server_project = 221;
-
         AddWorktree add_worktree = 222;
         AddWorktreeResponse add_worktree_response = 223;
 
         GetLlmToken get_llm_token = 235;
         GetLlmTokenResponse get_llm_token_response = 236;
+
+        LspExtSwitchSourceHeader lsp_ext_switch_source_header = 241;
+        LspExtSwitchSourceHeaderResponse lsp_ext_switch_source_header_response = 242; // current max
     }
 
     reserved 158 to 161;
@@ -2076,6 +2079,15 @@ message LspExtExpandMacroResponse {
     string expansion = 2;
 }
 
+message LspExtSwitchSourceHeader {
+    uint64 project_id = 1;
+    uint64 buffer_id = 2;
+}
+
+message LspExtSwitchSourceHeaderResponse {
+    string target_file = 1;
+}
+
 message SetRoomParticipantRole {
     uint64 room_id = 1;
     uint64 user_id = 2;

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!(