Configure KafkaAdmin for observation

[henriquejsza]

Closes gh-38830

This PR adds dedicated admin property groups for Kafka template and listener observation support. It allows the auto-configured KafkaTemplate and listener containers to use internal KafkaAdmin instances configured from spring.kafka.template.admin.* and spring.kafka.listener.admin.* when looking up the Kafka cluster id.

The listener container customization composes with a user-provided ContainerCustomizer so existing customization remains applied.

Tests:

• JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64 ./gradlew :module:spring-boot-kafka:test --tests org.springframework.boot.kafka.autoconfigure.KafkaAutoConfigurationTests --tests org.springframework.boot.kafka.autoconfigure.ConcurrentKafkaListenerContainerFactoryConfigurerTests
• JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64 ./gradlew :module:spring-boot-kafka:checkFormat :module:spring-boot-kafka:checkstyleMain :module:spring-boot-kafka:checkstyleTest
• git diff --check

[snicoll]

Thanks for the PR! I put it on hold as I'd like some clarity on what we're trying to achieve. I've asked on the linked issue.

[henriquejsza]

Thanks Stéphane, totally fair. I will pause this PR until the issue discussion defines the exact goal, then I will update it accordingly.

[henriquejsza]

Artem's follow-up in gh-38830 clarifies the intended scope: the existing auto-configured KafkaAdmin bean remains the injectable management bean, while the template/listener-specific instances are not beans and are used only for cluster-id lookup. That matches this PR's design.

I'll keep the code paused (and avoid rebasing the current conflicts) until you confirm that this resolves the scope concern. Once confirmed, I'll rebase and refresh the validation.