Implement support for mathematical programming in SQLFlow

[typhoonzero]

Please check out the design doc before starting to contribute: https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/design/programming_solver.md

TODO:

• [x] Add parser support to parse the programming extended syntax
• [x] Add IR struct to represent the programming statement
• [ ] Add sqlflow_submitter.solve() function to solve the programming problem as the Python API
  • [ ] support optflow job
  • [ ] support pyomo job
• [ ] Add code generators in go to generate code that calls sqlflow_submitter.solve()

[sneaxiy]

• [ ] Add sqlflow_submitter.solve() function to solve the programming problem as the Python API
  • [ ] support optflow job
  • [ ] support pyomo job
• [ ] Add code generators in go to generate code that calls sqlflow_submitter.solve()

@typhoonzero I would like to take part in these works.