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.
After this PR #9272, the `docs/build_help_zip.sh` will run failed.
This PR fix this issue.
But the help module still has some parse problem, I will fix it in next PR.
This CL mainly changes:
1. fix `docs/build_help_zip.sh` error
2. remove `sql-reference-v2` to `sql-reference`
3. modify build extension github action to run `docs/build_help_zip.sh`
