Implement pyalisa and model saver/loader in Python

[shendiaomo]

As the discussion of #2494 , SQLFlow would be made a pure compiler. This requires we support alisa in Python as a prerequisite.

• Implement a pyalisa module based on the Python DB API
• Implement a model save/load module in the Python package sqlflow.runtime
• Remove dependencies of goalisa and other database drivers in the Go codebase.