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

[renatonascalves]

Description

Bulk task already has an after_batch function but it's not pluggable. The idea here is to allow an implementer to plug its own callback to be executed after each batch.

https://github.com/alleyinteractive/wp-bulk-task/blob/main/src/class-bulk-task.php#L84-L124

Use Case

Alley devs: The need for this feature came from this.

Acceptance Criteria

• Allow a callback to be provided when the bulk task is being set up which will be called during the after_batch function. Define what values are available for passing to the callback (e.g., should we pass the current cursor position? the max number of records? etc).

[renatonascalves]

This is a similar request to #38