Define a unique identifier for each batched background migration

Steps needed to implement the proposed solution.

1. Define a unique identifier for each BBM (👈)
2. Include finalize_after timestamp in BBM data dictionary, with a rubocop to ensure it's presence.
3. Include finalized_by to store the migrations version which finalized the BBM.
4. Add a rubocop to ensure the presence of 'introduced_by' key in dictionary.
5. Run a cron to create a new issue after 'finalize_after' timestamp and add appropriate labels+assignee using introduced_by MR/issue.
6. Establish dependecy between new migrations and previously enqueued BBMs.
7. Add a rubocop to warn when the dependent BBMs are not finalized yet.