This patch supports utf8mb4 for mysql external table.
if someone needs a mysql external table with utf8mb4 charset, but only support charset utf8 right now.
When create mysql external table, it can add an optional propertiy "charset" which can set character fom mysql connection,
default value is "utf8". You can set "utf8mb4" instead of "utf8" when you need.
1. Support limit clause push down both odbc table and mysql table.
2. Code refactor of ODBC Scan Node, change `build_connect_string` and `query_string` from BE to FE to make it easily to modify
