[BYO-S3] Schema: assume-role fields (role ARN / external ID / pending identity)

### Task Summary

Part of #6870. Add the columns and table needed for the assume-role (Path B) credential model.

- **Two columns on `user_warehouse`** — `role_arn` (the IAM role Texera assumes) and `external_id`
(the per-warehouse ID tied to it), set only for AWS warehouses (null for Local).
- **`user_s3_pending_identity` table** — holds the external ID reserved for a user **before** their
warehouse exists, so they can set up their IAM role first: `uid` (PK, FK → `user`), `external_id`
(UUID), `created_at`.
- **jOOQ codegen** — regenerate for the new column(s) and table (and update `texera_ddl.sql`).

### Task Type
- [x] Other (feature implementation)