feat: add get_save_idxs_and_saved_subsystem

[AayushSabharwal]

This helps avoid duplicating code when adding symbolic save_idxs to solver libraries

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.