Playground on the cloud

[lhw362950217]

Our playground now is mainly focused on single user. Maybe we can move it to the cloud, then, the user can try it out with no configuration. To do this, we need to:

• use JupyterHub instead of Jupyter Notebook
• have a access control mechanism
• deploy a Kubernetes cluster

After that, users can logon JupyterHub and play SQLFlow in a virtual environment

[lhw362950217]

This feature has been implemented by sql-machine-learning/sqlflow#2408 sql-machine-learning/sqlflow#2413 sql-machine-learning/sqlflow#2526 sql-machine-learning/sqlflow#2881 sql-machine-learning/sqlflow#2937