BatchQueueController::process should accept a callable

Problem/Motivation

When the batch API was converted into OO syntax → and then into a service in into a service → the third parameter of BatchQueueController::process() was left as a string, as it was in Drupal 8.3 and before. Update this to be able to accept any callable for extra functionality.

Proposed resolution

Update the the code in core/lib/Drupal/Core/Batch/BatchQueueController.php and as well as other files as appropriate.

Remaining tasks

• Update the code

User interface changes

None

API changes

Extend Batch::addOperation()

Data model changes

None