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

[mengw15]

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

• Other (feature implementation)