Editor: support go to implementation (#7890)

Release Notes:

- Added "Go to implementation" support in editor.

crates/rpc/src/proto.rs

@@ -192,6 +192,8 @@ messages!(
     (GetReferencesResponse, Background),
     (GetTypeDefinition, Background),
     (GetTypeDefinitionResponse, Background),
+    (GetImplementation, Background),
+    (GetImplementationResponse, Background),
     (GetUsers, Foreground),
     (Hello, Foreground),
     (IncomingCall, Foreground),
@@ -312,6 +314,7 @@ request_messages!(
     (GetCodeActions, GetCodeActionsResponse),
     (GetCompletions, GetCompletionsResponse),
     (GetDefinition, GetDefinitionResponse),
+    (GetImplementation, GetImplementationResponse),
     (GetDocumentHighlights, GetDocumentHighlightsResponse),
     (GetHover, GetHoverResponse),
     (GetNotifications, GetNotificationsResponse),
@@ -388,6 +391,7 @@ entity_messages!(
     GetCodeActions,
     GetCompletions,
     GetDefinition,
+    GetImplementation,
     GetDocumentHighlights,
     GetHover,
     GetProjectSymbols,