[8.x] Optional param for chunk size on scout:import

[serpentblade]

Allow setting an optional chunk size when running a batch import.

This is particularly helpful if you'd like to index a model using a different batch size than the default set for the app. Ideally it may be nice to customize batch sizes per model, instead of a single shared config value to be used across the entire app.

This is a functional workaround as it exposes in the CLI an already available chunk size parameter in the Collection::serializable extension method.

This is very useful for reducing the overhead of queries/update index calls for simpler indexable models. (For example, being able to batch 10k at a time instead of the default 500). Same applies for potentially complex indexable items where you'd want to reduce memory overhead by indexing less than the default at a time.