Refactor repo contents API and add "contents-ext" API (#34822)

See the updated swagger document for details.

routers/api/v1/swagger/repo.go

@@ -331,6 +331,12 @@ type swaggerContentsListResponse struct {
 	Body []api.ContentsResponse `json:"body"`
 }
 
+// swagger:response ContentsExtResponse
+type swaggerContentsExtResponse struct {
+	// in:body
+	Body api.ContentsExtResponse `json:"body"`
+}
+
 // FileDeleteResponse
 // swagger:response FileDeleteResponse
 type swaggerFileDeleteResponse struct {