Project push rules: Add missing attributes for project_id field

Implementation plan

Now that we have organization_push_rules and group_push_rules we should start using push_rules as dedicated table for projects. To achieve this we should perform the following changes:

• Create a migration to make project_id to be NOT NULL (https://docs.gitlab.com/development/database/not_null_constraints/#add-a-not-null-column-to-an-existing-table).
• Create a migration to add a concurrent foreign key for project_id (https://docs.gitlab.com/development/database/foreign_keys/#adding-foreign-keys-in-migrations).
• Add sharding key for project_id (https://docs.gitlab.com/development/cells/#defining-a-sharding-key-for-all-organizational-tables).

Expectations

• Validate that push_rules table does not have records with NULL values for project_id.
• Create the corresponding migrations and necessary tests.
• Coordinate the corresponding database review.