[R-package]: add num_trees_per_iter, num_trees, and num_iter methods

[mayer79]

This PR adds to the R Booster class the following methods:

• num_trees_per_iter(),
• num_trees(), and
• num_iter()

Two functions from the C API are required:

• LGBM_BoosterNumModelPerIteration_R() via LGBM_BoosterNumModelPerIteration() and
• LGBM_BoosterNumberOfTotalModel_R() via LGBM_BoosterNumberOfTotalModel()

This additionally helps to solve #6380.

Note: With early stopping, the number of (relevant) iterations is kept in Booster$best_iter, while num_trees() and num_iter() are higher.

[jameslamb]

Thanks very much! I support adding these methods on the Booster and not also adding new exported functions for them. And the tests look great.

Will you please just consider my recommendation to use the language "trees" instead of "models"?

[jameslamb]

Looks great, thank you!

@jmoralez do you have time to also review this week? I'd like to hear your opinion on adding this to the R package's public API.

[jameslamb]

Thanks @jmoralez ! I'll look into the failing CI and merge this.