KAFKA-19558: Batch consumer group descriptions

[lh0156]

Summary

• Split consumer group describe requests into batches of at most 50 groups.
• Preserve the existing result aggregation and output behavior for individual and all-group descriptions.
• Keep the change scoped to describe operations, which avoids changing the existing offset reset behavior.

Tests

• RED: ConsumerGroupServiceTest.testDescribeConsumerGroupsIsBatched failed before the change because 101 group IDs were sent in one request.
• GREEN: ConsumerGroupServiceTest.testDescribeConsumerGroupsIsBatched
• ConsumerGroupServiceTest
• DescribeConsumerGroupTest.testDescribeAllExistingGroups
• Checkstyle, SpotBugs, and Spotless

[lh0156]

Revalidated the current head locally without changing the patch:

• ./gradlew :tools:test --tests org.apache.kafka.tools.consumer.group.ConsumerGroupServiceTest --tests org.apache.kafka.tools.consumer.group.DescribeConsumerGroupTest :tools:checkstyleMain :tools:checkstyleTest :tools:spotbugsMain :tools:spotlessCheck --no-build-cache --console=plain
• Result: BUILD SUCCESSFUL (112 actionable tasks)
• The batching regression test and the existing DescribeConsumerGroupTest integration coverage passed.

No code changes were made in this verification pass.