Add next desired sharding keys for portfolio_management

Add "next" desired sharding keys for feature category portfolio_management.

These tables have been identified as a cell local tables. All cell local tables require a sharding key or a desired sharding key

A desired sharding key has been automatically selected for these tables. These keys were chosen as the desired sharding keys, and we are labelling them as "next" because:

• this table does not have any belongs_to: relationship to a table that itself has a NOT NULL sharding key.
• however, this table has at least one belongs_to: relationship to a table that has a desired sharding key, that is yet to be backfilled.
• We work on the assumption that the desired sharding key on this parent association will be eventually backfilled, and when that is completed, the child table can inturn backfill from that parent association.

We use awaiting_backfill_on_parent: true to distinguish between "next" desired sharding keys and the normal desired sharding keys.

Additionally, gitlab_schema has been set to gitlab_main_cell for any tables didn't use this schema already. For these tables we have also added allow_cross_joins, allow_cross_transactions and allow_cross_foreign_keys. These will silence any existing violations, allowing the pipeline to pass without requiring further changes. In the future, we'll remove these allow_... statements and fix any violations as they arise. You can read more about this in the documentation for multiple databases

Reviewer, please confirm that:

• each of these tables can be classified as cell local
• the selected desired sharding key is appropriate
• the backfill configuration for the desired sharding key is correct

When you are finished, please request a review from the database maintainer suggested by Danger. If you have any questions or concerns, reach out to @tigerwnz or @manojmj.

You can see the POC MR of this approach here

This change was generated by gitlab-housekeeper using the Keeps::DetermineNextDesiredShardingKeyFeatureCategory keep.

To provide feedback on your experience with gitlab-housekeeper please comment in #442003.