Incremental solve options

[leonlan]

Incremental solve using callbacks during a solve. This is what incremental solve usually means in MIP solvers: adding cuts, lazy constraint generation, etc. This is not possible in CP-SAT. But there are other ways to do "incremental" solving, although in a bit higher-level fashion, which is mostly about performance (not rebuilding) and convenience (sensible interface).

Minimum

• [x] Passing solutions to warm start without rebuilding the model
  • [x] OR-Tools: clear hints
  • [x] CP Optimizer: doesn't matter I think
• [ ] Lexicographic/sequential objective: adding objective constraints and solving the next objective

Nice to haves

• [ ] ~Add constraints~
• [ ] ~Deleting constraints~
  • ~Clear the corresponding protobuf line https://stackoverflow.com/questions/68207910/how-to-modify-variables-and-constraints-of-an-existing-google-or-tools-constrain~