## What does this MR do and why?

Add and backfill project_id for approvals.

This table has a
[desired sharding key](https://docs.gitlab.com/ee/development/database/multiple_databases.html#define-a-desired_sharding_key-to-automatically-backfill-a-sharding_key)
configured ([view configuration](https://gitlab.com/gitlab-org/gitlab/-/blob/master/db/docs/approvals.yml)).

This merge request is the first step towards transforming the desired sharding key into a
[sharding key](https://docs.gitlab.com/ee/development/database/multiple_databases.html#defining-a-sharding-key-for-all-cell-local-tables).

This involves the following changes:

- Adding a new column that will serve as the sharding key (along with the relevant asynchronous index).
- Populating the sharding key when new records are created by adding a database function and trigger.
- Scheduling a [batched background migration](https://docs.gitlab.com/ee/development/database/batched_background_migrations.html)
  ...