[Group Hierarchy] Create database table

Create a new model called Namespaces::Descendants (plural on purpose) to store de-normalized descendant record ids.

Table structure:

      CREATE TABLE namespace_descendants (
        namespace_id bigint NOT NULL,
        self_and_descendant_group_ids bigint[] NOT NULL DEFAULT ARRAY[]::bigint[],
        all_project_ids bigint[] NOT NULL DEFAULT ARRAY[]::bigint[],
        traversal_ids bigint[] NOT NULL DEFAULT ARRAY[]::bigint[],
        outdated_at timestamp with time zone, -- outdated_at IS NULL means it's not outdated 
        calculated_at timestamp with time zone,
        PRIMARY KEY(namespace_id)
      )
      PARTITION BY HASH (namespace_id);

Consider list-partitioning when creating the table. The partitions should be created in the gitlab_partitions_static schema. Use 32 partitions.

You can see the migration from the PoC here: !134499 (diffs)

Note: for now don't add extra indexes to the table.