Make driver use options from the solver (#6930)

Make driver use options from the solver (#6930)

This PR removes explicit ownership of the options object from the main function and replaces it by an api::Solver object. To achieve this, it does a number of minor changes:
- api::Solver not takes a unique_ptr<Options> in its constructor
- CommandExecutor only holds a reference to (a unique ptr of) the api::Solver
- the main functions accesses options via the solver