[Feature Request] Implement SQLFlow Session

[Yancey0623]

Is your feature request related to a problem? Please describe.

For the current SQLFlow architecture, we should deploy SQLFlow with 1-1 mapping from Notebook server instance and the SQLFlow gRPC server instance, it's unprofitable to the scalability and optimizes the resource utilization. The new feature SQLFlow Session can deploy the two components with the 1-N mapping.

Some discussion: #399

Tasks:

• [Done] Design doc. add auth and session design #501
• [Done] add e2e test to ensure the all-in-one Docker image availability. Add e2e test  #529
• [Discussion] How to maintain the proto file in the seperate repos(sqlflow/pysqlflow) To avoid maintain sqlflow.proto file in the separated repo pysqlflow#69
• [Done] implement session in the server side (sqlflow) add session struct in server proto #530
• [Done] implement session in the client side (pysqlflow) add session struct in proto pysqlflow#71
• [Done] A tutorial to deploy SQLFlow cluster on k8s. run sqlflow hub on k8s #537

[tonyyang-svail]

I am closing this issue since having two proto definitions is working okay.