Implement `gsctl flexbuild` to support Flex

Add a subcommand `gsctl flexbuild` to gsctl, to build Flex with the commands listed here:
- https://github.com/alibaba/GraphScope/tree/main/flex

previously, these functions are implemented in
- https://github.com/alibaba/GraphScope/blob/main/flex/flexbuild

which is auto-generated by bash.ly, the source code is implemented:
- https://github.com/alibaba/GraphScope/tree/flexwip/flex/src

This task is a job of migration.