[SSO] Add design doc for group to role mapping for OIDC

[mtabebe]

Proposes JWT-based group-to-role sync for self-managed.

On connection, Materialize reads group claims from the JWT and grants/revokes role memberships accordingly. Track these using a dedicated sentinel grantor (MZ_JWT_SYNC_ROLE_ID) to distinguish sync-managed from manually-managed grants.

[github-actions]

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