Add and backfill multiple sharding keys for snippet_user_mentions.

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/snippet_user_mentions.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 three changes:

- Adding a new column that will serve as the sharding key (along with the relevant index and foreign key).
- 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)
  to set the sharding key for existing records.

Once the background migration has completed, a second merge request will be created to finalize the background
migration and validate the not null constraint.

Please review this merge
request from a ~backend perspective. The main thing we are looking to verify is that the added column and association
match the values specified by the [desired sharding key](https://gitlab.com/gitlab-org/gitlab/-/blob/master/db/docs/snippet_user_mentions.yml)
configuration and that backfilling the column from this other table makes sense in the context of this feature.

When you are finished, please:

1. Trigger the [database testing pipeline](https://docs.gitlab.com/ee/development/database/database_migration_pipeline.html)
   as instructed by Danger.
1. Request a review from the ~backend maintainer and ~database reviewer suggested by Danger.

If you have any questions or concerns, reach out to `@tigerwnz` or @shubhamkrai.

This merge request was generated by a once off keep implemented in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143774

This change was generated by
[gitlab-housekeeper](https://gitlab.com/gitlab-org/gitlab/-/tree/master/gems/gitlab-housekeeper)
using the Keeps::BackfillMultipleDesiredShardingKeySmallTable keep.

To provide feedback on your experience with `gitlab-housekeeper` please create an issue with the
label ~"GitLab Housekeeper" and consider pinging the author of this keep.

Changelog: other