Support ksqlDB window grace periods and EMIT FINAL

[minleejae]

ksqlDB queries fail on unparenthesized JOIN windows, grace periods and EMIT FINAL. Support these forms while retaining the existing parenthesized single-duration and asymmetric JOIN windows:

SELECT a.id FROM a JOIN b
WITHIN 1 HOUR GRACE PERIOD 15 MINUTES ON a.id = b.id
EMIT CHANGES;

SELECT item_id, SUM(quantity) FROM orders
WINDOW TUMBLING (SIZE 20 SECONDS, GRACE PERIOD 2 SECONDS)
GROUP BY item_id EMIT FINAL;

Reuse one duration production across JOIN, HOPPING/SESSION/TUMBLING and grace periods. KSQLWindow.Duration exposes the grace value and time unit, distinguishing an omitted grace period from zero. JOIN windows preserve their bracket style. PlainSelect.EmitMode exposes NONE/CHANGES/FINAL while the existing boolean API keeps its default behavior; model and deparser share EMIT rendering. A contextual alias check recognizes WITHIN durations without reserving ordinary aliases named within.

Validation:

• Full Gradle check and Maven verify.
• Original reproducers, all ten singular/plural time-unit spellings, three aggregation-window forms, symmetric/asymmetric JOIN windows, zero grace periods, LIMIT and following statements.
• AST fields, legacy builders, SQL/model/deparser round-trips, malformed syntax, keyword aliases and existing ksqlDB/ClickHouse/JOIN regression tests.
• Syntax follows the Confluent SELECT documentation.

Fixes #1752.