Issue-39: Allow an implementer-provided callback to be executed after each batch

[dustinpaluch]

Summary

Fixes #39

This pull request implements the feature request for allowing an implementer-provided callback to be executed after each bulk task batch. This feature aims to make the after_batch function pluggable, providing flexibility for different implementations and use cases.

Description

The bulk task functionality currently includes an after_batch function which is executed after each batch is processed. However, this function is not pluggable, limiting the ability to customize post-batch processing. This pull request addresses this limitation by allowing an implementer to plug in their own callback to be executed after each batch.

Reference: wp-bulk-task implementation

Use Case

This feature was requested to fulfill a specific need identified by Alley developers, as documented here.

Acceptance Criteria

• Implement a mechanism to allow a custom callback to be provided during the bulk task setup.
• The custom callback will be executed within the after_batch function.
• Define and document the parameters available to the custom callback, such as current cursor position and the maximum number of records.

Additional Notes

This feature request is similar to a previous request (see #38 for context).

For more details, refer to the original issue here.