Draft: BBO - Testing framework

Ref: #581295

What does this MR do and why?

This MR creates our first background operations to try out our new bbo framework.

Testing

Feature.enable(:schedule_background_operations)

Database::BackgroundOperation::CronEnqueueWorker.new.perform(
  'job_class_name' => 'UsersDeleteUnconfirmedSecondaryEmails',
  'table_name' => 'emails',
  'column_name' => 'id'
)