api/storage_service: Add get_release_version

$ curl -X GET --header "Accept: application/json"
"http://127.0.0.1:10000/storage_service/release_version"

"2.1.8"

api/storage_service.cc

@@ -72,8 +72,7 @@ void set_storage_service(http_context& ctx, routes& r) {
     });
 
     ss::get_release_version.set(r, [](const_req req) {
-        //TBD
-        return "";
+        return service::get_local_storage_service().get_release_version();
     });
 
     ss::get_schema_version.set(r, [](const_req req) {