Kafka gateway: serve IncrementalAlterConfigs (key 44) #4509

Follow-up to #3546. #4398 says this comes in a later pull request.

Summary

IncrementalAlterConfigs (44) is not served and is not advertised. Kafka 4.0 removed Admin.alterConfigs, and kafka-configs --alter sends key 44 only. A Java 4.x client or tool therefore cannot change any topic configuration through the gateway. The Java client and librdkafka fail locally, because the key is not advertised. They do not fall back to AlterConfigs (33).

Who hits it:

• kafka-configs --alter from Kafka 4.x, and Java 4.x Admin.incrementalAlterConfigs.
• ksqlDB, which alters its command topic at startup.
• MirrorMaker 2 into the gateway, which syncs topic configurations with key 44. Its source connector goes FAILED.
• kafka-ui, which picks its alter API from the broker version.

Depends on

• Kafka gateway: accept the topic configurations that frameworks send #4508 for keys other than retention.ms

Scope

• IncrementalAlterConfigs v0-1 for topic resources, with SET, DELETE, APPEND and SUBTRACT as Kafka defines them
• The same validation as AlterConfigs. With SASL on, the same manage_topics test
• validate_only runs every check and writes nothing
• Advertise the key, and update gateways/kafka/docs/CONFIGS.md

Acceptance criteria

• kafka-configs --alter --add-config retention.ms=... from Kafka 4.x changes the topic, and a describe shows the new value
• MirrorMaker 2 syncs topic configurations into the gateway, and its connector stays RUNNING

References

• Kafka 4.0 removes Admin.alterConfigs: upgrade.md:358
• kafka-configs uses incrementalAlterConfigs only: ConfigCommand.java:604-611
• MirrorMaker 2: MirrorSourceConnector.java:598-633
• ksqlDB: KsqlInternalTopicUtils.java