IAM Database Schema Design For Authentication

This issues aims to create a Auth DB schema design based on the considerations set out in https://gitlab.com/gitlab-org/gitlab/-/issues/555284+

• Document the schema for both Spanner (global/regional) and PostgreSQL (cell-local)
• Consider the original schema proposed in https://gitlab.com/gitlab-org/gitlab/-/issues/555284#note_2654690196 as a base point
• Address authentication vs authorization data separation.
• Define data flow between Rails monolith and IAM databases
• Document migration strategy and tooling approach

Acceptance Criteria:

• Complete DDL schema for both Spanner and PostgreSQL
• Data model supporting principals, identities, resources, capabilities, and grants
• Clear separation between authentication and authorization concerns
• Integration points with Topology Service defined
• Migration strategy documented with tooling selection