Draft: Fully shard the identities table

What does this MR do and why?

• Expose the identities table's sharding key
• Create a foreign key constraint (NOT VALID) on the sharding key column following https://docs.gitlab.com/development/database/foreign_keys/#adding-the-fk-constraint-not-valid
• Fix existing records following https://docs.gitlab.com/development/database/foreign_keys/#data-migration-to-fix-existing-records
• Prepare ASYNC foreign key validation following https://docs.gitlab.com/development/database/foreign_keys/#schedule-the-fk-to-be-validated

Follow up to !202814 (merged)

Addresses #578135

The table is sharded by identities#user_id.

SQL

DELETE FROM "identities" WHERE "identities"."id" >= 2 AND (NOT EXISTS (SELECT "users".* FROM "users" WHERE (users.id = identities.user_id)))

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