Tidy up collab-related signature help data (#14377)

Follow-up of https://github.com/zed-industries/zed/pull/12909

* Fully preserve LSP data when sending it via collab, and only strip it
on the client.
* Avoid extra custom request handlers, and extend multi LSP server query
protocol instead.


Release Notes:

- N/A

crates/collab/src/rpc.rs

@@ -642,10 +642,7 @@ impl Server {
                         app_state.config.openai_api_key.clone(),
                     )
                 })
-            })
-            .add_request_handler(user_handler(
-                forward_read_only_project_request::<proto::GetSignatureHelp>,
-            ));
+            });
 
         Arc::new(server)
     }