psql: Improve tab completion for ALTER TABLE on identity columns

psql: Improve tab completion for ALTER TABLE on identity columns

- Add tab completion for ALTER SEQUENCE … START …
- Add tab completion for ALTER COLUMN … SET GENERATED …
- Add tab completion for ALTER COLUMN … SET <sequence option>
- Add tab completion for ALTER COLUMN … ADD GENERATED … AS IDENTITY

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Matheus Alcantara <mths.dev@pm.me>
Discussion: https://www.postgresql.org/message-id/flat/87mta1jfax.fsf@wibble.ilmari.org