Add session struct in gRPC proto file #68
Description
As the design doc: https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/auth_design.md#session , we need to add session struct in gRPC proto.
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As the design doc: https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/auth_design.md#session , we need to add session struct in gRPC proto.