Convert `subscription_add_ons` table to be hard-coded in application code
The `subscription_add_ons` table is also a simple table with `id`, `name`, and `description` columns. Again, it has a unique index on the `name` column.
Similar to the [`plans` table](https://gitlab.com/gitlab-org/gitlab/-/issues/519892), we can either use a `name_uid` column strategy, or drop the table entirely.
### References
- https://docs.gitlab.com/development/cells/#static-data
- https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/decisions/014_clusterwide_syncing_in_cells_1_0/
## Action items
- [ ] Evaluate which approach to take. Preferred option is the drop the `subscription_add_ons` table
epic