[R-package] Expose start_iteration to dump/save/lgb.model.dt.tree

[mayer79]

The predict() function has a start_iteration argument. As in Python, we could expose this argument to

• lgb.dump() and Booster$dump_model(),
• lgb.save() and Booster$save_model(), and
• Booster$save_model_to_string().

Furthermore, we can also add it to lgb.model.dt.tree(), providing a straight-forward solution to #6380

[jameslamb]

Thank you, I support this!