Offline query output should have schema, not one string column

[vagetablechicken]

OpenMLDB/src/sdk/sql_cluster_router.cc

Line 2413 in b78fb44

// TODO(tobe): return result set if want to format the output

[vagetablechicken]

New place is

OpenMLDB/src/sdk/sql_cluster_router.cc

Line 2775 in 1317ce7

return ResultSetSQL::MakeResultSet({FORMAT_STRING_KEY}, {value}, status);

https://openmldb.ai/docs/zh/main/quickstart/sdk/java_sdk.html#statement doc should add a tip for it