Add BaseOperationWorker base class

Ref: #575416

What does this MR do and why?

This MR adds a base class to define operations which can be enqueued to our new BBO framework.

You will be able to define an operation as follow and queue it anywhere from the application:

module Gitlab
  module BackgroundMigration
    class DeleteExpiredTokens < BaseOperationWorker
      operation_name :delete_expired_tokens
      feature_category :system_access
      cursor :id

      def perform
        each_sub_batch do |sub_batch|
          sub_batch.where(revoked_at: ...cutoff_date).delete_all
      end
    end
  end
end

Note

It copy over the only the methods leveraging a cursor strategy from Gitlab::BackgroundMigration::BatchedMigrationJob.

This class does not implement distinct_each_batch for simplicity purpose, this can be added later on.